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/web/tool-switch.spec.ts

@@ -1,5 +1,5 @@
 import { test, expect, type Page } from '@playwright/test';
-import { clickMenuBarItem, clickMenuItem } from '../e2e/utils/element-tracker';
+import { clickMenuBarItem, clickMenuItemByText, stableShot } from '../e2e/utils/element-tracker';
 
 /**
  * Tool-switch e2e: eeschema Tools → "Switch to PCB Editor" (and the reverse)
@@ -38,9 +38,7 @@ async function switchTool(
   expectedTitle: RegExp
 ): Promise<void> {
   expect(await clickMenuBarItem(page, 'Tools'), 'Tools menubar item clickable').toBe(true);
-  await page.waitForTimeout(400); // popup render settle, same as tests/kicad specs
-
-  expect(await clickMenuItem(page, menuLabel), `"${menuLabel}" menu item clickable`).toBe(true);
+  await clickMenuItemByText(page, menuLabel);
 
   await page.waitForURL(expectedUrl, { timeout: 30000 });
   await waitForToolReady(page, expectedTitle);
@@ -60,10 +58,7 @@ test.describe('web app — tool switching', () => {
       /demo — PCB Editor/i
     );
 
-    await page.screenshot({
-      path: 'test-results/web-switch-sch-to-pcb.png',
-      scale: 'css',
-    });
+    await stableShot(page, 'web-switch-sch-to-pcb.png');
   });
 
   test('pcbnew → Switch to Schematic Editor navigates to eeschema', async ({ page }) => {
@@ -79,9 +74,6 @@ test.describe('web app — tool switching', () => {
       /demo — Schematic Editor/i
     );
 
-    await page.screenshot({
-      path: 'test-results/web-switch-pcb-to-sch.png',
-      scale: 'css',
-    });
+    await stableShot(page, 'web-switch-pcb-to-sch.png');
   });
 });