fix(renderer): WebGL longtask auto-fallback to canvas renderer (#83)

The xterm WebGL renderer can stall the main thread for hundreds of ms
under GPU pressure (driver hiccup, integrated-GPU memory pressure,
hardware-accelerated browser layers contending for the GPU). Symptom:
the page becomes intermittently unresponsive and Chrome eventually
shows the "Page Unresponsive" dialog. Today the only mitigation is
?nowebgl, which the user has to remember and re-apply on every load.

This patch installs a PerformanceObserver after WebGL init that
watches for sustained main-thread stalls and falls back to the DOM
renderer automatically:

- Threshold: 3 long tasks of >=200ms each within a 30-second window.
- 5-second grace period after init skips the noisy initial-load
  stalls so a slow first paint does not trip the guard.
- On trigger: dispose the WebGL addon, write a sticky disable to
  localStorage with a reason and timestamp, and refresh the terminal
  so the canvas renderer takes over without a page reload.
- Subsequent loads honor the sticky disable for 7 days, then auto-
  expire so users retry after a driver/Chrome update.
- Force re-enable any time with ?webgl=force (also clears the
  sticky entry).
- Existing ?nowebgl behaviour is unchanged.
- The same disable path is reused by the existing onContextLoss
  callback so a hard context loss also persists across reloads.

Files:
- src/web/public/app.js: _initWebGL onContextLoss now persists +
  schedules the watchdog; new _installWebGLLongTaskGuard and
  _disableWebGLSticky helpers.
- src/web/public/terminal-ui.js: WebGL init checks the sticky entry
  with 7-day expiry, honors ?webgl=force, threads sticky into
  skipWebGL alongside the existing mobile + ?nowebgl gates.

PerformanceObserver longtask is widely supported (Chromium, Edge);
the try/catch around .observe() makes Firefox/Safari (which lack the
longtask entry type) silently no-op and just keep WebGL.

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/web/public/terminal-ui.js

@@ -174,10 +174,36 @@ Object.assign(CodemanApp.prototype, {
     // but the 48KB/frame flush cap in flushPendingWrites() now prevents
     // oversized terminal.write() calls that triggered the stalls.
     // Disable with ?nowebgl URL param if GPU issues return.
+    // Auto-fallback: _initWebGL installs a long-task watchdog that disables
+    // WebGL sticky in localStorage after repeated GPU stalls (see app.js).
+    // Force re-enable after sticky disable with ?webgl=force.
     // Lazy-loaded: script downloaded only on desktop (saves 244KB on mobile).
     this._webglAddon = null;
-    const skipWebGL = MobileDetection.getDeviceType() !== 'desktop';
-    if (!skipWebGL && !new URLSearchParams(location.search).has('nowebgl')) {
+    const _params = new URLSearchParams(location.search);
+    if (_params.get('webgl') === 'force') {
+      try { localStorage.removeItem('codeman-webgl-disabled'); } catch {}
+    }
+    const _stickyDisabled = (() => {
+      try {
+        const raw = localStorage.getItem('codeman-webgl-disabled');
+        if (!raw) return false;
+        const { at } = JSON.parse(raw);
+        // Auto-expire after 7 days so we retry (driver may have been fixed)
+        if (Date.now() - at > 7 * 24 * 60 * 60 * 1000) {
+          localStorage.removeItem('codeman-webgl-disabled');
+          return false;
+        }
+        return true;
+      } catch { return false; }
+    })();
+    const skipWebGL =
+      MobileDetection.getDeviceType() !== 'desktop' ||
+      _params.has('nowebgl') ||
+      _stickyDisabled;
+    if (_stickyDisabled) {
+      console.log('[CRASH-DIAG] WebGL sticky-disabled from prior stalls — DOM renderer in use. Re-enable: ?webgl=force');
+    }
+    if (!skipWebGL) {
       if (typeof WebglAddon !== 'undefined') {
         this._initWebGL();
       } else {