WebGPU pixel look: GPU colours the world, CPU only rare cells (t88 phase 2a)

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

docs/agent-log.md

@@ -4,6 +4,22 @@ What agents have done, newest first. Add an entry for every piece of work: the d
 note or request it was for, what changed, the files touched, the tests run, and anything left
 open or not verified. Keep each entry short; the git history has the details.
 
+## 2026-10-27: t88 phase 2a, WebGPU pixel look (Claude Code + Codex)
+
+- New `src/render/gpu/` (device, textures, colour compute, glow blur, draw) renders the pixel look
+  on a `#gpu` canvas under `#game`, which becomes a transparent overlay in WebGPU mode. The GPU
+  colours plain cells (background, materials with noise and grain, loose shade, fire, water,
+  ALPHA, digp shake, incandescence, lens, fog); the CPU colours only rare cells (built blocks,
+  screw/slope neighbours, bars, flyers, ghosts) through the extracted per-cell code
+  (`src/render/cell.js`, `src/render/objects.js`). World arrays upload per visible rectangle with
+  `queue.writeTexture`; light is still `computeLight` on the CPU, multiplied on the GPU.
+  `SET.renderer` = webgpu | legacy (Settings). The smooth look still uses the WebGL2 path.
+- Screenshots legacy vs WebGPU: mean abs RGB error 0.112 / 0.062 / 0.034 at zoom 12/6/2, no pixel
+  off by more than 8. Factory scene drawWorld at zoom 2: 24.55 -> 9.98 ms (overrides 1.81, uploads
+  2.39, CPU light 5.59; GPU time 0.06 ms). Real loop zoom 2 CPU p50/p95: 31.7/35.3 -> 15.4/16.7 ms.
+- Tests: `python3 test/run_all.py --browser` all passed (new `test/t_gpu.py`). Not checked:
+  Firefox WebGPU (no adapter in Playwright Firefox on Linux).
+
 ## 2026-10-27: t88 phase 1, serving and capability checks (Claude Code + Codex)
 
 - The game must be served with COOP/COEP (README has nginx and Caddy lines; opening from disk is