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>

README.md

@@ -193,6 +193,22 @@ npx playwright test menu    # Menu tests only
 
 See [tests/README.md](tests/README.md) for test documentation.
 
+### Screenshots
+
+CI's Linux render is the source of truth for baseline screenshots. On each `main`
+push, CI compares its render against the committed baselines and posts the diff
+(plus the runtime-perf numbers) to Discord. To update baselines after an intended
+render change, promote a CI run's render — only meaningfully-changed images
+restage, so it stays churn-free:
+
+```bash
+cd tests
+npm run screenshots:check                          # local gate: current vs baselines
+npm run screenshots:promote -- --run <ci-run-id>   # adopt a CI run's render, then commit
+```
+
+See [tests/tools/screenshots/README.md](tests/tools/screenshots/README.md).
+
 ## Current Status
 
 - **wxWidgets WASM**: Core widgets working (menus, dialogs, grids, trees, OpenGL)