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/eeschema-crosshair.spec.ts

@@ -223,14 +223,14 @@ test.describe('Eeschema crosshair modes', () => {
 
         await page.mouse.move(probe.x, probe.y);
         await page.waitForTimeout(600);
-        const shotSmall = await page.screenshot({ path: 'test-results/eeschema-crosshair-00-small.png', scale: 'device' });
+        const shotSmall = await page.screenshot({ path: 'test-results/eeschema-crosshair-00-small.png', scale: 'css' });
 
         // click 1 -> full-window
         await clickAndSettle();
         await expect.poll(tooltipNow, {
             message: 'one click should advance to Full-Window Crosshairs', timeout: 6000,
         }).toContain('Full-Window Crosshairs');
-        const shotFull = await page.screenshot({ path: 'test-results/eeschema-crosshair-01-full.png', scale: 'device' });
+        const shotFull = await page.screenshot({ path: 'test-results/eeschema-crosshair-01-full.png', scale: 'css' });
         expect((await compareScreenshots(page, shotSmall, shotFull, diffRegion)).diffPixels,
             'full-window crosshair should visibly differ from the small crosshair').toBeGreaterThan(200);
 
@@ -239,7 +239,7 @@ test.describe('Eeschema crosshair modes', () => {
         await expect.poll(tooltipNow, {
             message: 'second click should advance to 45 Degree Crosshairs', timeout: 6000,
         }).toContain('45 Degree Crosshairs');
-        const shot45 = await page.screenshot({ path: 'test-results/eeschema-crosshair-02-45.png', scale: 'device' });
+        const shot45 = await page.screenshot({ path: 'test-results/eeschema-crosshair-02-45.png', scale: 'css' });
         expect((await compareScreenshots(page, shotFull, shot45, diffRegion)).diffPixels,
             '45-degree crosshair should visibly differ from the full-window crosshair').toBeGreaterThan(200);
 
@@ -248,7 +248,7 @@ test.describe('Eeschema crosshair modes', () => {
         await expect.poll(tooltipNow, {
             message: 'third click should cycle back to Small crosshairs', timeout: 6000,
         }).toContain('Small crosshairs');
-        await page.screenshot({ path: 'test-results/eeschema-crosshair-03-small-again.png', scale: 'device' });
+        await page.screenshot({ path: 'test-results/eeschema-crosshair-03-small-again.png', scale: 'css' });
 
         const realErrors = testLogger.errors.filter((error: string) => !error.includes('favicon'));
         expect(realErrors).toEqual([]);