diff --git a/docs/shell-split-harness.md b/docs/shell-split-harness.md
new file mode 100644
index 00000000..2af75a0a
--- /dev/null
+++ b/docs/shell-split-harness.md
@@ -0,0 +1,211 @@
+# Shell split verification: the harness
+
+Split out of `docs/shell-split-verification.md` when that file hit the 250-line
+cap. That file covers **what makes a green split wrong**; this one covers the
+tooling that catches it — the trace harness, its `--prefix` mode, the
+regression fixtures, and two traps that have each cost a revert.
+
+Read `docs/shell-split-verification.md` first: the hazards there decide whether
+the harness can prove anything about your target at all.
+
+### `trace_shell_split.sh` — the run, made routine
+
+`meta/scripts/trace_shell_split.sh` (added 2026-08-16) does the stubbed run for
+you. It shadows ~30 mutating binaries (`sudo`, `pacman`, `systemctl`, `adb`,
+`nft`, `mount`, `mkinitcpio`, `reboot`, …) with `PATH` stubs that record each
+call and exit 0, runs the script, and prints a diffable trace: exit status,
+stubbed calls in order with arguments, stdout, stderr.
+
+```bash
+# baseline, in a detached worktree at the pre-split commit
+meta/scripts/trace_shell_split.sh <script> --out /tmp/before.txt
+# after the split
+meta/scripts/trace_shell_split.sh <script> --out /tmp/after.txt
+diff /tmp/before.txt /tmp/after.txt
+```
+
+It was verified against both bugs above: the `set -e` function tail shows up as
+exit 1 with later calls missing, and the nameref as `circular name reference`
+in stderr despite exit 0. A fixture calling `systemctl` left the unit untouched.
+
+Four things to watch:
+
+- **A silent stub produces an identical, meaningless trace.** This is the
+  trap. `du` stubbed to print nothing makes `size=$(du -sk x)` empty, so every
+  `((size > 0))` is false, every guarded branch is skipped, and two such traces
+  match perfectly while exercising none of the code you changed. Give
+  value-producing commands a plausible output: `--stub 'du=4096\t/some/dir'`.
+  Measured on a fixture: the empty stub never reached the guarded
+  `sudo rm -rf`; the valued stub printed `BRANCH TAKEN` and captured it. Ask of
+  every trace: _did this actually go down the branch I touched?_
+- **Stubs always exit 0.** A script that branches on a real failure status
+  takes a path it would not take live. Hand-write a stub for those and say so
+  in the split's evidence file.
+- **Anything not stubbed runs for real.** `git`, `curl`, `wget` and `makepkg`
+  are deliberately excluded from the default set, because many scripts read git
+  state harmlessly and a blanket stub would change what they see. Grep the
+  target for network and build verbs, then pass them:
+  `--stub git,curl,makepkg`.
+- **`pkill` is NOT stubbed and `--prefix` cannot contain it** — signals are not
+  filesystem writes. `install_leechblock.sh:217` pkills every Chromium-family
+  browser before seeding, so tracing it closes the user's browsers (measured:
+  two probe runs fired it, harmless only because none was open). Grep a target
+  for `pkill`/`killall`/`systemctl stop` and pass `--stub pkill` first. The
+  mirror image also escapes the prefix: `setup_thorium_startup.sh` _launches_ a
+  browser mid-run ("Browser launched with PID: …"). Process spawning is not a
+  file write either — check `ps` after tracing anything that starts a program.
+
+### `--prefix` — scripts whose job is placing files
+
+Shell redirections (`cat > /etc/modprobe.d/x.conf`) are builtins, so no `PATH`
+stub can intercept them. Stub `cp`/`chmod`/`ln` and the trace is nearly empty;
+don't, and the run rewrites the live system. `--prefix` (added 2026-08-16)
+resolves that in two layers — see `meta/scripts/lib/trace_prefix.sh`:
+
+```bash
+# $HOME-rooted destinations: no bwrap, no root needed
+meta/scripts/trace_shell_split.sh install_usage_monitoring.sh --prefix /tmp/pfx-before
+
+# hardcoded absolutes: bind each LEAF directory
+meta/scripts/trace_shell_split.sh nvidia_troubleshoot.sh --prefix /tmp/pfx-nv \
+  --bind-abs /etc/modprobe.d --bind-abs /etc/X11 --stub git,lsmod,nvidia-smi
+```
+
+**Every run needs its own empty prefix**, hence the `-before`/`-after` naming.
+Reusing one leaves the first run's files in place, so a dropped write stops
+showing up entirely — measured: the fixture pair below diffs _clean_ on a
+reused prefix. The harness refuses a non-empty prefix rather than deleting it.
+
+`HOME` **and every `XDG_*` base** are redirected. Exporting the XDG vars
+explicitly is not belt-and-braces: `install_leechblock.sh` reads
+`XDG_DATA_HOME` directly and then runs `rsync -a --delete` at it, so inheriting
+the caller's real value would delete live data.
+
+The trace gains a `=== files written (prefix)` section listing every file with
+its size and a content hash. **That section is the point**: without it a
+dropped `cat >` shows up as nothing at all — same exit status, same stubbed
+calls, same stdout — and the traces match while one lost a file. Measured on
+the `dropped_write_{before,after}.sh` fixture pair: with the manifest the lost
+`.timer` is a one-line diff; stripped, the two traces are byte-identical.
+
+Six traps, each of which cost a debugging round:
+
+- **`/usr/bin` cannot be bound at all.** `--bind-abs /usr/bin` overlays the
+  directory the sandbox needs to exec anything (`bwrap: execvp bash`). A script
+  whose last act replaces a binary there (`install_pacman_wrapper.sh:304`,
+  `ln -sf … /usr/bin/pacman`) therefore **stops at that line with exit 1** on
+  EPERM. Stable across runs, so still a usable baseline — but it proves nothing
+  at or after the failing line, so keep those statements in the entry script.
+  The scanner misses `/usr/bin` here: it follows variables into `cat >`/`cp`,
+  so a literal `ln -sf` (or `cp "$f" "$f.backup.$(date …)"`) escapes it.
+- **Bind leaf directories, never `/etc`.** Binding `/etc` wholesale shadows
+  `/etc/passwd` and `/etc/os-release`. Measured: `$SUDO_USER` lookup collapsed
+  to `/home//pyroveil`, `/etc/profile` became "No such file or directory", and
+  a nested `/etc/X11` bind was masked so its write vanished from the manifest.
+  All three look like a broken split. The scanner therefore never emits a bare
+  `/etc`. A `--bind-abs` naming a **file** (`/etc/profile`) is bound
+  file-over-file, seeded with a copy of the original — bwrap cannot mount a
+  directory over a file. The copy matters because a script that greps a config
+  before appending would otherwise read an empty file and branch wrong. Note a
+  file bind cannot cover new _siblings_: `cp "$f" "$f.backup.<stamp>"` still
+  EPERMs, which is what truncates `nvidia_troubleshoot.sh`'s trace at step 3.
+- **`require_root` truncates a stubbed run to nothing.** `lib/common.sh` does
+  `exec sudo "$0" "$@"`; under the `sudo` stub that records one line and exits
+  0 — a three-line trace that diffs clean against _any_ split. `--bind-abs`
+  runs under `bwrap --unshare-user --uid 0`, so `$EUID` is 0 and the script
+  proceeds for real. Without a bind there is no uid 0 and the truncation is
+  silent, so check the trace actually entered the code you moved.
+- **Unquoted heredocs bake the prefix into the artifact.**
+  `install_usage_monitoring.sh:222` uses `<< SCRIPT`, so `$HOME` interpolates
+  at write time. Hashes are taken over content with the prefix normalized to
+  `@PREFIX@`; without that, a `mktemp -d` prefix makes every run differ.
+- **A timestamp the target embeds still varies.** `nvidia_troubleshoot.sh`
+  writes `$(date)` into its config, so its hash changes every run. Don't freeze
+  the clock and don't fall back to comparing sizes — diff the written files'
+  _contents_ with the date pattern normalized away. That still catches a
+  dropped or altered line, which a size comparison does not.
+
+Anything absolute that was _not_ bound fails read-only/EPERM rather than
+succeeding, so Decision 6 holds by construction. `--prefix` **refuses to run**
+when the target writes to absolute paths and no `--bind-abs` was given; the
+scan follows variables (`MODPROBE_DIR="/etc/modprobe.d"` → `cat >"$CONFIG_FILE"`),
+because a literal-only scan produced a silent empty manifest.
+
+Verified untouched after the runs above: `~/.config/systemd/user`,
+`~/.local/bin`, `~/.local/share`, `/etc/modprobe.d` (identical listings), and
+`systemctl --user list-unit-files | grep -c usage-report` unchanged at 2.
+
+`block_compulsive_opening.sh` remains out of scope: `install_all` copies the
+running script into `/usr/local/bin`, and the handoff records that a bad split
+there breaks three daily-use apps plus the pacman rewrap hook.
+
+### Regression fixtures
+
+`meta/scripts/fixtures/` holds runnable reproductions of every trap above, so a
+harness change can be checked instead of trusted:
+
+| Fixture                           | Reproduces                                      |
+| --------------------------------- | ----------------------------------------------- |
+| `set_e_function_tail.sh`          | exit 1, later calls missing from the trace      |
+| `circular_nameref.sh`             | exit 0, correct stdout, warning only in stderr  |
+| `valued_stub_branch.sh`           | the silent-stub trap (`--stub du` vs `du=4096`) |
+| `dropped_write_{before,after}.sh` | a split that silently loses one write           |
+| `xdg_destinations.sh`             | writes addressed via `XDG_*`, not `$HOME`       |
+
+A lint fix once broke one of these invisibly: adding a trailing `echo` to
+satisfy SC2034 made _that_ echo the function's last statement, so the fixture
+exited 0 and reproduced nothing while still "passing". Re-run the fixtures
+after touching them, and check the exit status is still the one documented.
+
+**Run the baseline from the script's own directory** when it reads relative
+paths. `fresh-install/main.sh` reads `aur_packages.txt` and
+`pacman_packages.txt` from the cwd; tracing it from elsewhere dies at the first
+read and looks exactly like a broken split.
+
+Note it does not need a `main()`, which matters: none of the remaining
+near-miss shell targets (`install_usage_monitoring.sh`,
+`install_pacman_wrapper.sh`, `disk_cleanup_check.sh`, `fresh-install/main.sh`)
+have one, so `verify_shell_split.sh` cannot check any of them.
+
+Note the knock-on: renaming the nameref local hides the array from shellcheck
+(SC2034), because it is then only ever named as a bare word. Pass the name
+through a variable (`CODE_FILES_MAP="LANG_CODE_FILES"`) so the reference is
+real, rather than suppressing.
+
+### Verify through every entry point, especially symlinks
+
+Adding a `source "$SCRIPT_DIR/lib/..."` line to `meta/lint_python.sh` broke
+`./lint_python.sh` from the repo root, because that root path is a **symlink**
+into `meta/` (so are `pyproject.toml`, `requirements.txt`, `run.sh`, `.fvmrc`).
+`SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"` does not resolve
+symlinks — `dirname` is string manipulation — so it became the repo root, where
+`lib/` does not exist, and `set -euo pipefail` turned that into an instant exit.
+
+It was invisible because every check used `bash meta/lint_python.sh`, the one
+path that cannot fail. Before splitting a script, ask how else it is reachable:
+
+```bash
+find . -maxdepth 2 -type l -lname '*<script>*'   # symlinks pointing at it
+grep -rn '<script>' .github/workflows/ .pre-commit-config.yaml meta/run.sh
+```
+
+The fix is the pattern already used elsewhere in this repo:
+
+```bash
+SCRIPT_DIR="$(dirname "$(readlink -f "${BASH_SOURCE[0]}")")"
+```
+
+Note the pre-split script had the same unresolved `SCRIPT_DIR`; it only fed
+`PROJECT_ROOT`, so the bug was latent. Adding a `source` line is what turns a
+quiet wrong value into a hard failure — which is exactly what a split does.
+
+### Do not make a clean file shfmt-dirty
+
+157 of 298 `linux_configuration` shell files already fail `shfmt` and there is
+no `.editorconfig` or shfmt pre-commit hook, so `shell_check.sh`'s `run_linters`
+exits 1 on the full tree regardless. That is not licence to add more: check
+`shfmt -d <file>` before and after a split. `analyze_repo.sh` was clean and a
+split made it dirty over one double blank line.
+
+`shfmt -w` also **deletes comment banners** it considers stray — it removed a
+`#====` STEP header. Re-count the banners after running it.
diff --git a/docs/shell-split-recipes.md b/docs/shell-split-recipes.md
index 692a9bfc..8bf38370 100644
--- a/docs/shell-split-recipes.md
+++ b/docs/shell-split-recipes.md
@@ -23,7 +23,8 @@ enforcement script that mutates the live system or the phone.**
   state — `analyze_repo.sh` passed every static check plus the stub and still
   aborted at runtime. See **`docs/shell-split-verification.md`**, which now
   holds every "the split was green and still wrong" lesson, including the
-  `set -e` function-tail trap and the flag that looked safe and was not.
+  `set -e` function-tail trap and the flag that looked safe and was not, and
+  **`docs/shell-split-harness.md`** for running `trace_shell_split.sh` itself.
 
 ### Use `${BASH_SOURCE[0]}`, not `$0`
 
diff --git a/docs/shell-split-verification.md b/docs/shell-split-verification.md
index 51c42604..fba47af5 100644
--- a/docs/shell-split-verification.md
+++ b/docs/shell-split-verification.md
@@ -69,204 +69,10 @@ the system, and diff the generated artifacts against a detached worktree at the
 pre-split commit. That comparison is what proved the fix — same file set, every
 text artifact byte-identical.
 
-### `trace_shell_split.sh` — the run, made routine
+## The harness and the fixtures
 
-`meta/scripts/trace_shell_split.sh` (added 2026-08-16) does the stubbed run for
-you. It shadows ~30 mutating binaries (`sudo`, `pacman`, `systemctl`, `adb`,
-`nft`, `mount`, `mkinitcpio`, `reboot`, …) with `PATH` stubs that record each
-call and exit 0, runs the script, and prints a diffable trace: exit status,
-stubbed calls in order with arguments, stdout, stderr.
-
-```bash
-# baseline, in a detached worktree at the pre-split commit
-meta/scripts/trace_shell_split.sh <script> --out /tmp/before.txt
-# after the split
-meta/scripts/trace_shell_split.sh <script> --out /tmp/after.txt
-diff /tmp/before.txt /tmp/after.txt
-```
-
-It was verified against both bugs above: the `set -e` function tail shows up as
-exit 1 with later calls missing, and the nameref as `circular name reference`
-in stderr despite exit 0. A fixture calling `systemctl` left the unit untouched.
-
-Four things to watch:
-
-- **A silent stub produces an identical, meaningless trace.** This is the
-  trap. `du` stubbed to print nothing makes `size=$(du -sk x)` empty, so every
-  `((size > 0))` is false, every guarded branch is skipped, and two such traces
-  match perfectly while exercising none of the code you changed. Give
-  value-producing commands a plausible output: `--stub 'du=4096\t/some/dir'`.
-  Measured on a fixture: the empty stub never reached the guarded
-  `sudo rm -rf`; the valued stub printed `BRANCH TAKEN` and captured it. Ask of
-  every trace: _did this actually go down the branch I touched?_
-- **Stubs always exit 0.** A script that branches on a real failure status
-  takes a path it would not take live. Hand-write a stub for those and say so
-  in the split's evidence file.
-- **Anything not stubbed runs for real.** `git`, `curl`, `wget` and `makepkg`
-  are deliberately excluded from the default set, because many scripts read git
-  state harmlessly and a blanket stub would change what they see. Grep the
-  target for network and build verbs, then pass them:
-  `--stub git,curl,makepkg`.
-- **`pkill` is NOT stubbed and `--prefix` cannot contain it** — signals are not
-  filesystem writes. `install_leechblock.sh:217` pkills every Chromium-family
-  browser before seeding, so tracing it closes the user's browsers (measured:
-  two probe runs fired it, harmless only because none was open). Grep a target
-  for `pkill`/`killall`/`systemctl stop` and pass `--stub pkill` first. The
-  mirror image also escapes the prefix: `setup_thorium_startup.sh` _launches_ a
-  browser mid-run ("Browser launched with PID: …"). Process spawning is not a
-  file write either — check `ps` after tracing anything that starts a program.
-
-### `--prefix` — scripts whose job is placing files
-
-Shell redirections (`cat > /etc/modprobe.d/x.conf`) are builtins, so no `PATH`
-stub can intercept them. Stub `cp`/`chmod`/`ln` and the trace is nearly empty;
-don't, and the run rewrites the live system. `--prefix` (added 2026-08-16)
-resolves that in two layers — see `meta/scripts/lib/trace_prefix.sh`:
-
-```bash
-# $HOME-rooted destinations: no bwrap, no root needed
-meta/scripts/trace_shell_split.sh install_usage_monitoring.sh --prefix /tmp/pfx-before
-
-# hardcoded absolutes: bind each LEAF directory
-meta/scripts/trace_shell_split.sh nvidia_troubleshoot.sh --prefix /tmp/pfx-nv \
-  --bind-abs /etc/modprobe.d --bind-abs /etc/X11 --stub git,lsmod,nvidia-smi
-```
-
-**Every run needs its own empty prefix**, hence the `-before`/`-after` naming.
-Reusing one leaves the first run's files in place, so a dropped write stops
-showing up entirely — measured: the fixture pair below diffs _clean_ on a
-reused prefix. The harness refuses a non-empty prefix rather than deleting it.
-
-`HOME` **and every `XDG_*` base** are redirected. Exporting the XDG vars
-explicitly is not belt-and-braces: `install_leechblock.sh` reads
-`XDG_DATA_HOME` directly and then runs `rsync -a --delete` at it, so inheriting
-the caller's real value would delete live data.
-
-The trace gains a `=== files written (prefix)` section listing every file with
-its size and a content hash. **That section is the point**: without it a
-dropped `cat >` shows up as nothing at all — same exit status, same stubbed
-calls, same stdout — and the traces match while one lost a file. Measured on
-the `dropped_write_{before,after}.sh` fixture pair: with the manifest the lost
-`.timer` is a one-line diff; stripped, the two traces are byte-identical.
-
-Six traps, each of which cost a debugging round:
-
-- **`/usr/bin` cannot be bound at all.** `--bind-abs /usr/bin` overlays the
-  directory the sandbox needs to exec anything (`bwrap: execvp bash`). A script
-  whose last act replaces a binary there (`install_pacman_wrapper.sh:304`,
-  `ln -sf … /usr/bin/pacman`) therefore **stops at that line with exit 1** on
-  EPERM. Stable across runs, so still a usable baseline — but it proves nothing
-  at or after the failing line, so keep those statements in the entry script.
-  The scanner misses `/usr/bin` here: it follows variables into `cat >`/`cp`,
-  so a literal `ln -sf` (or `cp "$f" "$f.backup.$(date …)"`) escapes it.
-- **Bind leaf directories, never `/etc`.** Binding `/etc` wholesale shadows
-  `/etc/passwd` and `/etc/os-release`. Measured: `$SUDO_USER` lookup collapsed
-  to `/home//pyroveil`, `/etc/profile` became "No such file or directory", and
-  a nested `/etc/X11` bind was masked so its write vanished from the manifest.
-  All three look like a broken split. The scanner therefore never emits a bare
-  `/etc`. A `--bind-abs` naming a **file** (`/etc/profile`) is bound
-  file-over-file, seeded with a copy of the original — bwrap cannot mount a
-  directory over a file. The copy matters because a script that greps a config
-  before appending would otherwise read an empty file and branch wrong. Note a
-  file bind cannot cover new _siblings_: `cp "$f" "$f.backup.<stamp>"` still
-  EPERMs, which is what truncates `nvidia_troubleshoot.sh`'s trace at step 3.
-- **`require_root` truncates a stubbed run to nothing.** `lib/common.sh` does
-  `exec sudo "$0" "$@"`; under the `sudo` stub that records one line and exits
-  0 — a three-line trace that diffs clean against _any_ split. `--bind-abs`
-  runs under `bwrap --unshare-user --uid 0`, so `$EUID` is 0 and the script
-  proceeds for real. Without a bind there is no uid 0 and the truncation is
-  silent, so check the trace actually entered the code you moved.
-- **Unquoted heredocs bake the prefix into the artifact.**
-  `install_usage_monitoring.sh:222` uses `<< SCRIPT`, so `$HOME` interpolates
-  at write time. Hashes are taken over content with the prefix normalized to
-  `@PREFIX@`; without that, a `mktemp -d` prefix makes every run differ.
-- **A timestamp the target embeds still varies.** `nvidia_troubleshoot.sh`
-  writes `$(date)` into its config, so its hash changes every run. Don't freeze
-  the clock and don't fall back to comparing sizes — diff the written files'
-  _contents_ with the date pattern normalized away. That still catches a
-  dropped or altered line, which a size comparison does not.
-
-Anything absolute that was _not_ bound fails read-only/EPERM rather than
-succeeding, so Decision 6 holds by construction. `--prefix` **refuses to run**
-when the target writes to absolute paths and no `--bind-abs` was given; the
-scan follows variables (`MODPROBE_DIR="/etc/modprobe.d"` → `cat >"$CONFIG_FILE"`),
-because a literal-only scan produced a silent empty manifest.
-
-Verified untouched after the runs above: `~/.config/systemd/user`,
-`~/.local/bin`, `~/.local/share`, `/etc/modprobe.d` (identical listings), and
-`systemctl --user list-unit-files | grep -c usage-report` unchanged at 2.
-
-`block_compulsive_opening.sh` remains out of scope: `install_all` copies the
-running script into `/usr/local/bin`, and the handoff records that a bad split
-there breaks three daily-use apps plus the pacman rewrap hook.
-
-### Regression fixtures
-
-`meta/scripts/fixtures/` holds runnable reproductions of every trap above, so a
-harness change can be checked instead of trusted:
-
-| Fixture                           | Reproduces                                      |
-| --------------------------------- | ----------------------------------------------- |
-| `set_e_function_tail.sh`          | exit 1, later calls missing from the trace      |
-| `circular_nameref.sh`             | exit 0, correct stdout, warning only in stderr  |
-| `valued_stub_branch.sh`           | the silent-stub trap (`--stub du` vs `du=4096`) |
-| `dropped_write_{before,after}.sh` | a split that silently loses one write           |
-| `xdg_destinations.sh`             | writes addressed via `XDG_*`, not `$HOME`       |
-
-A lint fix once broke one of these invisibly: adding a trailing `echo` to
-satisfy SC2034 made _that_ echo the function's last statement, so the fixture
-exited 0 and reproduced nothing while still "passing". Re-run the fixtures
-after touching them, and check the exit status is still the one documented.
-
-**Run the baseline from the script's own directory** when it reads relative
-paths. `fresh-install/main.sh` reads `aur_packages.txt` and
-`pacman_packages.txt` from the cwd; tracing it from elsewhere dies at the first
-read and looks exactly like a broken split.
-
-Note it does not need a `main()`, which matters: none of the remaining
-near-miss shell targets (`install_usage_monitoring.sh`,
-`install_pacman_wrapper.sh`, `disk_cleanup_check.sh`, `fresh-install/main.sh`)
-have one, so `verify_shell_split.sh` cannot check any of them.
-
-Note the knock-on: renaming the nameref local hides the array from shellcheck
-(SC2034), because it is then only ever named as a bare word. Pass the name
-through a variable (`CODE_FILES_MAP="LANG_CODE_FILES"`) so the reference is
-real, rather than suppressing.
-
-### Verify through every entry point, especially symlinks
-
-Adding a `source "$SCRIPT_DIR/lib/..."` line to `meta/lint_python.sh` broke
-`./lint_python.sh` from the repo root, because that root path is a **symlink**
-into `meta/` (so are `pyproject.toml`, `requirements.txt`, `run.sh`, `.fvmrc`).
-`SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"` does not resolve
-symlinks — `dirname` is string manipulation — so it became the repo root, where
-`lib/` does not exist, and `set -euo pipefail` turned that into an instant exit.
-
-It was invisible because every check used `bash meta/lint_python.sh`, the one
-path that cannot fail. Before splitting a script, ask how else it is reachable:
-
-```bash
-find . -maxdepth 2 -type l -lname '*<script>*'   # symlinks pointing at it
-grep -rn '<script>' .github/workflows/ .pre-commit-config.yaml meta/run.sh
-```
-
-The fix is the pattern already used elsewhere in this repo:
-
-```bash
-SCRIPT_DIR="$(dirname "$(readlink -f "${BASH_SOURCE[0]}")")"
-```
-
-Note the pre-split script had the same unresolved `SCRIPT_DIR`; it only fed
-`PROJECT_ROOT`, so the bug was latent. Adding a `source` line is what turns a
-quiet wrong value into a hard failure — which is exactly what a split does.
-
-### Do not make a clean file shfmt-dirty
-
-157 of 298 `linux_configuration` shell files already fail `shfmt` and there is
-no `.editorconfig` or shfmt pre-commit hook, so `shell_check.sh`'s `run_linters`
-exits 1 on the full tree regardless. That is not licence to add more: check
-`shfmt -d <file>` before and after a split. `analyze_repo.sh` was clean and a
-split made it dirty over one double blank line.
-
-`shfmt -w` also **deletes comment banners** it considers stray — it removed a
-`#====` STEP header. Re-count the banners after running it.
+Everything about _running_ the check — `trace_shell_split.sh`, the `--prefix`
+mode for scripts whose job is placing files, the regression fixtures, the
+entry-point and `shfmt` traps — moved to
+**`docs/shell-split-harness.md`** when this file hit the same 250-line cap it
+exists to serve. This file stays the _why_; that one is the _how_.