Give gatelock's test job a display, so its real-Tk tests actually run

Headless, the viewport, fit-check and keyboard tests skip -- 25 of them --
and take their coverage with them, which is why this job went red the moment
they landed: 100% is the bar and _fitcheck drops to 47%, _keyboard to 18%.

Skipping is the worse half. These tests exist because Tk's own focus
delivery and canvas scrolling cannot be mocked -- mocking them is exactly how
a viewport that scrolled itself passed every check for a week -- so a green
run that skipped them would be reporting nothing about the thing they guard.
xvfb-run gives the runner a display, which is what every local run already
used.

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

.github/workflows/gatelock-tests.yml

@@ -29,5 +29,13 @@ jobs:
       - name: Install dependencies
         run: pip install -r requirements.txt
 
+      # The viewport, fit-check and keyboard tests drive real Tk: focus
+      # delivery, canvas scrolling and widget heights are Tk's own behaviour,
+      # and mocking them is what let a viewport that scrolled itself pass
+      # every check for a week. Headless they skip, silently taking coverage
+      # with them -- so the runner gets a display instead.
+      - 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