test(wasm): coroutine crash reproduction harness + per-engine runner

Investigation scaffolding for the Chrome-only KiCad coroutine renderer crash.
Adds isolated reproduction probes exercising the coroutine/Asyncify/fiber layer
under KiCad-like conditions, runnable in BOTH Firefox and system Chrome.

- tests/playwright-coroutine.config.ts + test:coroutine:firefox|chrome npm
  scripts: run the coroutine specs in Firefox AND system Chrome (the old e2e
  config only used bundled Chromium, which never reproduced the crash).
- tests/apps/standalone/coroutine-pthread/: no-wx + pthreads reproduction probes
  (fiber-in-main, nested invoke_/dynCall boundaries, RunMainStack, embind,
  main-loop/rAF activation) + worker_dom_stub.js for wx+pthreads builds.
- tests/apps/Makefile.wasm: coroutine-pthread{,-main,-nested,-nested-ex,-wx,
  -embind,-mainloop} targets.
- scripts/common/shims/diagnostics.js: add EM_ASYNC_JS handleSleep enter/wake
  tracking (DIAG_SLEEP) to detect nested-async at the crash.

Findings (details in research notes): every isolated factor so far — direct /
nested / RunMainStack fiber, wx event loop + all 13 scenarios incl EM_ASYNC_JS,
pthreads, and main-loop/rAF activation — runs CLEAN in system Chrome. The
coroutine/Asyncify layer is exonerated; GL/WebGL is the remaining untested factor
(next). The reliable FF-pass/Chrome-fail repro is still the KiCad pcbnew e2e.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

tests/package.json

@@ -12,7 +12,9 @@
     "test:kicad:firefox": "npm run setup:kicad && playwright test --config=playwright-kicad.config.ts --project=firefox",
     "test:kicad:chrome": "npm run setup:kicad && playwright test --config=playwright-kicad.config.ts --project=chromium --headed",
     "test:kicad": "npm run test:kicad:firefox",
-    "test:kicad:headed": "npm run test:kicad:chrome"
+    "test:kicad:headed": "npm run test:kicad:chrome",
+    "test:coroutine:firefox": "playwright test --config=playwright-coroutine.config.ts --project=firefox",
+    "test:coroutine:chrome": "playwright test --config=playwright-coroutine.config.ts --project=chromium --headed"
   },
   "devDependencies": {
     "@playwright/test": "^1.40.0",