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/web/tool-switch.spec.ts

@@ -62,7 +62,7 @@ test.describe('web app — tool switching', () => {
 
     await page.screenshot({
       path: 'test-results/web-switch-sch-to-pcb.png',
-      scale: 'device',
+      scale: 'css',
     });
   });
 
@@ -81,7 +81,7 @@ test.describe('web app — tool switching', () => {
 
     await page.screenshot({
       path: 'test-results/web-switch-pcb-to-sch.png',
-      scale: 'device',
+      scale: 'css',
     });
   });
 });

tests/web/tools-open.spec.ts

@@ -81,7 +81,7 @@ test.describe('web app — tool open paths', () => {
       page,
     }) => {
       await bootAndAssert(page, tc);
-      await page.screenshot({ path: `test-results/web-${tool}.png`, scale: 'device' });
+      await page.screenshot({ path: `test-results/web-${tool}.png`, scale: 'css' });
     });
   }
 });