Give the test job a display, so the real-Tk tests run instead of failing

This job has been red since the keyboard tests landed: they drive real Tk --
widget heights and the focus ring come from the font engine, and no mock can
answer what they ask -- and a runner has no X display, so they failed with
"no display name and no $DISPLAY" rather than skipping. Thirteen tests,
including the two that assert the gate fits 1366x768, were reporting nothing
about the code.

xvfb-run gives them the display they need, which is what every local run of
these tests already used.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BsXnTHyuuCUZsqetTaajcF

.github/workflows/python-tests.yml

@@ -24,5 +24,12 @@ jobs:
       - name: Install dependencies
         run: pip install -r requirements.txt
 
+      # The gate's keyboard and layout tests drive real Tk: widget heights and
+      # the focus ring come from the font engine, and no mock can answer what
+      # they ask. Without a display they do not skip -- they fail, which is how
+      # this job stayed red while the tests themselves were fine.
+      - name: Install a virtual X display
+        run: sudo apt-get update && sudo apt-get install -y xvfb
+
       - name: Run tests with coverage
-        run: python -m pytest -v
+        run: xvfb-run -a -s "-screen 0 1600x1200x24" python -m pytest -v