fix(tests): enforce screenshot gate; drop shadow dir, flaky retinascale, 12 stale

Finalize the screenshot review system after the first Linux re-baseline
(that run showed 355/356 stable — the placeholder floor is fine as-is):

- ENFORCE the gate: `screenshots:check --fail-on-change` now fails the build on any
  changed/added/removed vs baselines (it still posts the drift report first, so the
  failure is actionable; a real render change → re-promote). The generic "CI failed"
  notice is suppressed for gate failures so we don't double-post on drift.
- Exclude retinascale-01-loaded: a fullPage HiDPI test whose captured height + DPR
  scaling vary run-to-run (~60% inter-run diff) — a flaky test, not render noise.
  IGNORE_SCREENSHOTS in config.ts; compare/promote/gen-manifest skip it; baseline removed.
- Kill the baseline-dir shadowing: drop e2e/baseline-screenshots/ from BASELINE_DIRS +
  delete its 3 files (grid-tab-final, wxgrid-controls, wxgrid-dedicated-page) that
  duplicated names in baseline-screenshots/ with different bytes.
- Prune 12 stale baselines (renamed/removed specs: wizard-01..04, gerbview-wizard-01..04,
  zoom-pl_editor-*, popup-03-palette).
- Delete the dead compare-screenshots.sh / update-baseline-screenshots.sh.
- Regenerate screenshot-manifest.json (355 entries).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/tools/screenshots/gen-manifest.ts

@@ -19,7 +19,7 @@
  */
 import * as fs from 'fs';
 import * as path from 'path';
-import { BASELINE_DIRS, MANIFEST_PATH, type Manifest } from './config';
+import { BASELINE_DIRS, MANIFEST_PATH, isIgnored, type Manifest } from './config';
 
 const CHROMIUM = 'chromium-swiftshader';
 const FIREFOX = 'firefox-llvmpipe';
@@ -78,7 +78,7 @@ function listBaselines(root: string): string[] {
     for (const dir of BASELINE_DIRS) {
         const abs = path.join(root, dir);
         if (!fs.existsSync(abs)) continue;
-        for (const f of fs.readdirSync(abs)) if (f.toLowerCase().endsWith('.png')) names.add(f);
+        for (const f of fs.readdirSync(abs)) if (f.toLowerCase().endsWith('.png') && !isIgnored(f)) names.add(f);
     }
     return [...names].sort();
 }