Adds an arch-tests job on archlinux:base-devel so the pacman wrapper,
makepkg wrapper and hosts-guard tests finally run somewhere. They need
an Arch userland and root, which a disposable container provides.
Four more tests still referenced pre-reorganisation paths and could
never find their targets; repairing them took the container set from
two passing to five. All five were verified with --network=none, so
the job installs nothing beyond git (base-devel ships no git, and
actions/checkout needs it).
Also removes the silent-failure mode behind the earlier
music_parallelism confusion. The test now derives its temp-worktree
path from the script's real position instead of hardcoding the depth,
checks up front that ../../lib/common.sh resolves to the stub, and has
the stub write a marker asserted after every run. Previously a
reorganisation made the script source the real
/usr/local/lib/common.sh, so the test measured the live system and
failed as though the daemon were broken.
Three tests stay out, each for a different reason, documented in the
workflow: test_i3blocks_efficiency.sh needs strace and could not be
verified locally (containers here have no DNS);
test_hosts_guard_pacman_integration.sh fails a real assertion worth
investigating; test_security_hardening.sh asserts live host state and
can never pass in a fresh container.
