Run CI on a WSL2 runner with the RTX 4070 (#283)

stein2 should stop carrying the full e2e run. The new runner lives in
the WSL2 distro game-maker-ci on melektaus-atx and uses the same image
and fonts. Its GPU sits behind /dev/dxg and Mesa's d3d12 driver, which
headless chromium cannot reach, so the harness gets a second way to
start the browser.

- GAME_MAKER_BROWSER=wsl starts an invisible Xvfb and an X11 chromium
  on ANGLE/GL. Without the variable, stein2 keeps the headless Vulkan
  launch exactly as before.
- The harness logs the renderer and stops on software rendering.
  Under WSL a missing d3d12 driver falls back to llvmpipe, which still
  offers WebGL 2 and would pass unnoticed.
- The CI image gains xvfb. The font set is unchanged: fc-list gives
  the same 149 fonts with the same hash.
- ci.yml runs both jobs on the new label game-maker-wsl. The stein2
  runner keeps game-maker-ci, so moving back is one line per job.

Findings:
- Wayland on weston reached the GPU too, but drew at about 40 fps and
  let the editor clock stall in long runs (GM_STEPS=1-17: 2 failed).
- Vulkan through a self-built dzn caps ANGLE at ES 2.0 because
  VK_EXT_provoking_vertex is missing, so chromium drops the GPU.
- The flaky check in e2e 41 (#282) fails more often under WSL: two of
  three solo runs were red.

Tests:
- ./check.sh: 195 files, 2823 tests passed
- WSL container, GM_STEPS=1-17: ALL PASSED, 553 checks
- WSL container, full run: 1589 of 1590, the one FAIL is #282
- WSL container, smokes: 24 / 19 / 21 checks
- stein2 headless, e2e 39+40: ALL PASSED, 100 checks

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tools/ci/Dockerfile

@@ -10,11 +10,14 @@ FROM node:22-bookworm
 # chromium + the Vulkan drivers: the e2e harness starts chromium with --use-angle=vulkan and has
 # no software fallback. The render device itself comes from the runner's container.options;
 # without it chromium falls back to llvmpipe and gets WebGL 1.0 only.
+# xvfb serves the other GPU path, GAME_MAKER_BROWSER=wsl (tools/lib/browser.mjs): under WSL2 the
+# GPU is /dev/dxg behind Mesa's d3d12 driver, which headless chromium cannot reach, an X11 one can.
 # php-cli serves api/index.php, which every tool loads and saves its project through.
 # The fonts are the host's set on purpose: the checks measure text widths to find click points,
 # so a different set moves them.
 RUN apt-get update && apt-get install -y --no-install-recommends \
       chromium mesa-vulkan-drivers libvulkan1 \
+      xvfb \
       php-cli php-mbstring \
       fonts-dejavu-core fonts-droid-fallback fonts-noto-color-emoji fonts-noto-mono \
       fonts-texgyre fonts-urw-base35 \