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/kicad/3d-viewer.spec.ts

@@ -118,7 +118,7 @@ test.describe('3D viewer from pcbnew', () => {
         await waitForPcbnew(page);
 
         await loadBoard(page, testLogger);
-        await page.screenshot({ path: `test-results/3d-viewer-00-board-loaded.png`, scale: 'device' });
+        await page.screenshot({ path: `test-results/3d-viewer-00-board-loaded.png`, scale: 'css' });
 
         const glBefore = await countGlCanvases(page);
         console.log(`[TEST] glcanvas count before opening 3D viewer: ${glBefore}`);
@@ -129,7 +129,7 @@ test.describe('3D viewer from pcbnew', () => {
         // the scene and render a few progressive passes.
         await page.waitForTimeout(5000);
 
-        await page.screenshot({ path: `test-results/3d-viewer-${DEMO.name}.png`, scale: 'device' });
+        await page.screenshot({ path: `test-results/3d-viewer-${DEMO.name}.png`, scale: 'css' });
 
         // Read the 3D viewer canvas (the newest glcanvas) directly from its backing
         // store: copy it onto a 2D canvas with drawImage and sample pixels. This is
@@ -312,7 +312,7 @@ test.describe('3D viewer from pcbnew', () => {
             return all.find((id) => !before.includes(id)) ?? all[all.length - 1] ?? null;
         }, winsBefore);
         expect(winId, 'the 3D viewer should open a new top-level window').toBeTruthy();
-        await page.screenshot({ path: 'test-results/3d-viewer-titlebar.png', scale: 'device' });
+        await page.screenshot({ path: 'test-results/3d-viewer-titlebar.png', scale: 'css' });
 
         // It must have a real DOM title bar (the frames-only fix covers the viewer).
         const bar = page.locator(`#${winId} .window-titlebar`);