feat(tests): screenshot regression + Discord review, perf-tracked

New tooling in tests/tools/screenshots/ (TypeScript via tsx):
- compare.ts: one pixelmatch engine (AA-excluded), connected-component
  "where to look" boxes, old|new+boxes|heatmap triptych, per-engine floors.
- promote.ts: churn-free updater — overwrite a baseline only when decoded
  pixels differ beyond the floor, copying CI bytes verbatim (no re-encode
  churn); pulls a CI run via `gh run download` or a local --from dir.
- post-discord.ts: always-on CI-on-main report (SHA + e2e status + the
  track-only runtime-perf table), then screenshot triptychs, batched +
  size-capped + flood-collapsed + 429-aware.
- perf-report.ts: perf table with Δ vs the previous main run (via gh).
- changelog.ts: no-build git-history baseline differ (Discord trigger B).
- noise.ts / gen-manifest.ts: calibration + manifest generation.

CI wiring:
- wasm-build.yml: post-test step runs the gate + report on the already-
  produced test-results (no extra build); report-only (continue-on-error),
  posts only on push to main, inert without DISCORD_WEBHOOK_URL.
- ci-ubicloud.yml: secrets: inherit (pass the webhook through).
- screenshot-changelog.yml: ~30s no-build changelog on baseline changes.

screenshot-manifest.json: canonical 354-name set + best-effort engine tags
(313 chromium-swiftshader / 41 firefox-llvmpipe).

Normalize scale:'device'->'css' across 18 spec files (no-op at CI DSF=1)
so committed baselines are uniformly css-scaled.

Design: CI's Linux render is the single source of truth; no pinned
container (accept rare env drift -> re-promote); dev commits via promote.
Replaces the byte-cmp compare-screenshots.sh + file-size-proxy
update-baseline-screenshots.sh (kept for now until the first re-baseline).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/package.json

@@ -42,13 +42,24 @@
     "test:asyncify:firefox": "playwright test --config=playwright-asyncify.config.ts --project=firefox",
     "test:asyncify:chrome": "playwright test --config=playwright-asyncify.config.ts --project=chromium --headed",
     "test:asyncify:safari": "playwright test --config=playwright-asyncify.config.ts --project=webkit",
-    "test:asyncify:all": "npm run test:asyncify:firefox && npm run test:asyncify:safari && npm run test:asyncify:chrome"
+    "test:asyncify:all": "npm run test:asyncify:firefox && npm run test:asyncify:safari && npm run test:asyncify:chrome",
+    "screenshots:check": "tsx tools/screenshots/compare.ts",
+    "screenshots:promote": "tsx tools/screenshots/promote.ts",
+    "screenshots:noise": "tsx tools/screenshots/noise.ts",
+    "screenshots:report": "tsx tools/screenshots/post-discord.ts",
+    "screenshots:changelog": "tsx tools/screenshots/changelog.ts",
+    "screenshots:manifest": "tsx tools/screenshots/gen-manifest.ts"
   },
   "devDependencies": {
     "@playwright/test": "^1.40.0",
     "@types/node": "^24.10.1",
+    "@types/pixelmatch": "^5.2.6",
+    "@types/pngjs": "^6.0.5",
     "esbuild": "^0.28.0",
+    "pixelmatch": "^5.3.0",
+    "pngjs": "^7.0.0",
     "serve": "^14.2.0",
+    "tsx": "^4.22.4",
     "typescript": "^5.9.3",
     "yjs": "^13.6.31"
   }