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/TESTING.md

@@ -110,6 +110,15 @@ python3 tools/shot.py shots/silo.png --setup tools/scenes/clear-sky.js \
   `tools/scenes/clear-sky.js` clears a flat work area and sets `FLOOR`, `BX`, `BY` and `CAM`. Set
   `CAM = [x, y]` to frame the shot.
 - The HUD and the cursor previews are hidden unless you pass `--hud` or `--previews`.
+- `--renderer webgpu|legacy` selects the pixel renderer (default WebGPU). WebGPU screenshots
+  fail clearly when no device is available; Smooth still uses WebGL2.
+- `test/t_gpu.py` compares the rendered composite against Legacy for ordinary cells, built pieces,
+  ghosts, fog and all thermal palettes, and checks world replacement, resize, timing and device loss.
+  World-pixel tests use `worldPixelContext()` to read both stacked canvases. The helper
+  presents the retained GPU result again before copying, because canvas textures expire after
+  presentation ([WebGPU specification](https://gpuweb.github.io/gpuweb/#abstract-opdef-expire-the-current-texture),
+  [Mozilla implementation notes](https://bugzilla.mozilla.org/show_bug.cgi?id=1965377)). Tests that change
+  static `noiseB` in place set `GPU.world = null` before drawing to upload their new fixture.
 - `--look smooth` for the smooth look. Test in the pixel look unless you are checking graphics.
 - `--freeze` draws the last frame at a fixed time and frame count, so animations are caught at
   the same moment. Two shots of the same scene then match pixel for pixel, and a before / after
@@ -193,7 +202,7 @@ python3 tools/shot.py shots/silo.png --setup tools/scenes/clear-sky.js \
   simulation work per frame and number of ticks. p50/p95/p99 use interpolated sorted samples;
   five warm-up frames are excluded. Zero-tick frames correctly have zero simulation time.
   JSON also records the smooth renderer's adaptive resolution scale: compare it as well as time.
-  Comparisons require identical options, print all timing changes, and mark >10% regressions;
+  Comparisons accept subsets of the baseline scenes, looks and zooms, with identical world/viewport dimensions and run duration, print all timing changes, and mark >10% regressions;
   tiny timings and tails can be noisy, and a zero baseline has no meaningful percent change.
 
   Performance runs launch Chromium with
@@ -212,3 +221,15 @@ python3 tools/shot.py shots/silo.png --setup tools/scenes/clear-sky.js \
 - `tools/experiments/factory_cost.js`: what running factories cost the simulation.
 - `tools/experiments/flow.js`: belt and screw conveyor throughput in grains a second.
 - `tools/experiments/worldgen.js`: material counts and timings of world generation.
+
+WebGPU Phase 2a: Colour measures the CPU override fill (including loose objects and ghosts).
+`frame_ms` is the complete paused-frame CPU call (including overlays and UI).
+Upload measures strided visible-array writes and the override/light uploads, and Blit measures
+GPU command submission. `override_cells` records the selected CPU cells; GPU timing is a
+completed timestamp sample for that configuration across colour, bloom and drawing when `timestamp-query` is
+available. The timestamp excludes uploads and the CPU lighting pass. Lighting remains on the
+CPU. The GPU canvas draws the world; the transparent game canvas holds the existing overlays.
+
+The comparison fixture is `tools/scenes/t88-renderer.js`. Capture with `--freeze --zoom 12`
+(and 6 or 2), `--viewport 1920x1080`, and each `--renderer`. It includes terrain, water, fire,
+magma, fog, a tree, flat/slope belts, screws, pipe, mold, bar and blueprint ghost.