Describe the WSL2 runner and its browser mode (#283)

CI moved from stein2 to a runner in a WSL2 distro on melektaus-atx.

- Development: ci.yml asks for game-maker-wsl, the stein2 runner keeps
  game-maker-ci as the way back.
- The GPU per runner: the render node on stein2, /dev/dxg with Xvfb
  and ANGLE/GL under WSL2 (GAME_MAKER_BROWSER=wsl).
- The renderer check that stops a run on software rendering.

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

Development.md

@@ -39,8 +39,10 @@ in software that is ruinous: measured here on an idle page over 20 s, software W
 a core at 8 fps**, the same page on the Intel HD 630 via ANGLE/Vulkan costs **69% at 56 fps**. The
 full suite takes about four minutes that way. `tools/lib/browser.mjs` therefore always asks for the GPU and has
 **no software fallback** — on a host without one, chromium should fail loudly rather than melt the
-machine down. (`--use-angle=gl` and `--use-gl=egl` measure cheaper still, but get no WebGL context at
-all, so the app never renders.)
+machine down. (`--use-angle=gl` and `--use-gl=egl` measure cheaper still, but a headless chromium
+gets no WebGL context with them at all, so the app never renders.) Every run also names its renderer
+in the log (`renderer: …`) and stops if that is a software one such as llvmpipe or SwiftShader:
+under WSL2 a missing GPU driver does not fail by itself, llvmpipe still offers WebGL 2 there.
 
 **Leave the full run to CI.** Every push runs the whole proof on the runner (see *Continuous
 integration*), so locally take only what the change needs: the gate, plus the step that covers it —
@@ -284,10 +286,27 @@ Every push runs the whole proof on the Gitea runner: `.gitea/workflows/ci.yml` i
 seeders need (`PUT` does not create a project, `POST` hands out a random id). Every tool takes its
 server from `GAME_MAKER_BASE_URL`.
 
-The job runs in the image from `tools/ci/Dockerfile` under the label `game-maker-ci`: chromium
-with the Vulkan drivers, `php-cli`, and the host's font packages, because the checks measure text
-widths to find click points. The **render device is not in this repo** — `/dev/dri/renderD128`
-and a 1 GB `/dev/shm` come from `container.options` in the runner's own `config.yaml`, which
-applies to every job on that runner. Without them chromium falls back to llvmpipe, which gets
-WebGL 1.0 only. Measured in the job container: `ANGLE (Intel, Vulkan 1.3.230, HD Graphics 630)`,
-WebGL 2.0, and the same 1027 checks as here.
+The jobs run in the image from `tools/ci/Dockerfile`: chromium with the Vulkan drivers and
+`xvfb`, `php-cli`, and the host's font packages, because the checks measure text widths to find
+click points. `ci.yml` asks for the label **`game-maker-wsl`** (#283): a runner in a WSL2 distro
+on a Windows machine with an RTX 4070, so this host no longer carries the suite. The runner on
+this host is still registered with the label `game-maker-ci` and the same image; pointing both
+`runs-on` back at that label moves the runs here again.
+
+The **GPU is not in this repo**. It comes from `container.options` in each runner's own
+`config.yaml`, which applies to every job on that runner:
+
+- **Here:** `/dev/dri/renderD128` and a 1 GB `/dev/shm`. Measured in the job container:
+  `ANGLE (Intel, Vulkan 1.3.230, HD Graphics 630)`, WebGL 2.0.
+- **Under WSL2:** `/dev/dxg`, the Windows driver libraries from `/usr/lib/wsl` and the same
+  `/dev/shm`, plus `GAME_MAKER_BROWSER=wsl` from the runner's `envs`. There is no DRM render node
+  and no Vulkan there, and a headless chromium gets no WebGL at all. In this mode
+  `tools/lib/browser.mjs` starts an invisible Xvfb and an X11 chromium on ANGLE/GL, which reaches
+  the GPU through Mesa's d3d12 driver: `ANGLE (Microsoft Corporation, D3D12 (NVIDIA GeForce RTX
+  4070), OpenGL 4.2)`, WebGL 2.0, 60 frames a second. A Wayland chromium reached the GPU as well,
+  but let the editor's clock stall in long runs.
+  The suite takes about 40% longer there than here, almost all of it in the modules that read
+  back many pixels (drawing, tilemaps, shaders).
+
+Without these options chromium falls back to software rendering, and the harness stops at the
+renderer check.