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>

.github/workflows/screenshot-changelog.yml

@@ -0,0 +1,41 @@
+name: screenshot-changelog
+
+# Discord trigger B: when a push to main changes committed baseline PNGs, post an
+# old | new+boxes | heatmap triptych per changed baseline (ADDED image / REMOVED
+# title too). No build, no GPU — it only diffs two git revisions of PNGs that are
+# already CI-rendered, so it runs in ~30s. Complements the re-render drift gate in
+# wasm-build.yml (which catches un-blessed renders); this is the human-facing feed
+# of intentional baseline updates as they land.
+
+on:
+  push:
+    branches: [main]
+    paths:
+      - 'tests/baseline-screenshots/**.png'
+      - 'tests/e2e/baseline-screenshots/**.png'
+
+concurrency:
+  group: screenshot-changelog-${{ github.ref }}
+  cancel-in-progress: false
+
+jobs:
+  changelog:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          # HEAD^ is needed to diff the tip commit's baseline changes. (A push of
+          # multiple commits reports only the tip commit's baseline diff — fine for
+          # a changelog; baseline refreshes are single commits in practice.)
+          fetch-depth: 2
+      - uses: actions/setup-node@v4
+        with:
+          node-version: 20
+      - name: Install screenshot tooling deps
+        working-directory: tests
+        run: npm ci
+      - name: Post baseline changelog to Discord
+        working-directory: tests
+        env:
+          DISCORD_WEBHOOK_URL: ${{ secrets.DISCORD_WEBHOOK_URL }}
+        run: npm run screenshots:changelog