test(determinism): deterministic waits + stableShot screenshots; drop blind sleeps/ifs/retries

Make the Playwright e2e + kicad suites deterministic so screenshot flake stops
tracing to timing races.

- Blind page.waitForTimeout -> condition waits (expect.poll, web-first
  assertions, waitUntil) + readiness helpers (waitForWxApp, waitForCanvasApp).
  Remaining sleeps are documented interaction dwells (annotated).
- Defensive "if element exists" branches -> loud asserts; label-fallback chains
  -> normalized clickMenuItemByText. First-run wizard for/if loops removed by
  seeding calculator/gerbview/pcbnew HTMLs.
- Screenshots: new stableShot(page, name) settles the render in-page (canvas
  hash over rAF) then writes a raw PNG to test-results/ for the existing offline
  gate (tools/screenshots vs baseline-screenshots). Replaces toHaveScreenshot,
  which did inline compare + its own baselines and had decoupled the specs from
  the real gate. scale:'css' pinned.
- retries: 0 in both configs.
- Guard: tests/tools/lint-determinism.ts (npm run lint:determinism) bans blind
  sleeps / toHaveScreenshot / inline retries / swallowed catches in specs;
  documented exceptions carry a marker. Rules in tests/TESTING.md.

Assertions, coverage, and renders unchanged (semantic-equivalence reviewed;
captures pixel-identical modulo inherent timer/timestamp/3d-raytrace variance).
Both suites green at retries:0 (e2e 340, kicad 92); ~35-61% faster.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BVX1pHMvRPYHdp6ZfEawrk

tests/e2e/bitmask.spec.ts

@@ -1,16 +1,15 @@
 // Bitmap Masking Tests - wxMask and transparent bitmap drawing
-import { test, expect, tryLoadApp } from './utils/fixtures';
+import { test, expect, waitForWxApp } from './utils/fixtures';
+import { stableShot } from './utils/element-tracker';
 
 test.describe('Bitmap Masking Tests', () => {
 
   test('Bitmap masking renders correctly', async ({ page, testLogger }) => {
     await page.goto('/standalone/bitmask/bitmask_test.html');
-    const loaded = await tryLoadApp(page);
+    await waitForWxApp(page);
 
-    await page.waitForTimeout(500);
-    await page.screenshot({ path: 'test-results/bitmask.png', fullPage: true });
+    await stableShot(page, 'bitmask.png', { fullPage: true });
 
-    expect(loaded, 'Bitmap masking app should load').toBe(true);
     expect(testLogger.errors.filter(e => !e.includes('favicon'))).toHaveLength(0);
   });