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/app.js

@@ -617,14 +617,57 @@ class CodemanApp {
       this._webglAddon = new WebglAddon.WebglAddon();
       this._webglAddon.onContextLoss(() => {
         console.error('[CRASH-DIAG] WebGL context LOST — falling back to canvas renderer');
-        this._webglAddon.dispose();
+        _crashDiag.log('WEBGL_LOST');
+        this._disableWebGLSticky('context-lost');
+        this._webglAddon?.dispose();
         this._webglAddon = null;
       });
       this.terminal.loadAddon(this._webglAddon);
       console.log('[CRASH-DIAG] WebGL renderer enabled');
+      this._installWebGLLongTaskGuard();
     } catch (_e) { /* WebGL2 unavailable — canvas renderer used */ }
   }
 
+  /**
+   * Watch for sustained main-thread stalls that indicate WebGL/GPU trouble.
+   * After 3 long tasks (>=200ms each) within 30s, dispose the WebGL addon and
+   * persist a sticky disable so subsequent reloads also use the DOM renderer.
+   * 5s grace period skips initial-load stalls. Force-re-enable: ?webgl=force.
+   */
+  _installWebGLLongTaskGuard() {
+    if (typeof PerformanceObserver === 'undefined' || this._webglLongTaskObserver) return;
+    const installedAt = performance.now();
+    const recent = [];
+    try {
+      this._webglLongTaskObserver = new PerformanceObserver((list) => {
+        if (!this._webglAddon) return;
+        const now = performance.now();
+        if (now - installedAt < 5000) return;
+        for (const entry of list.getEntries()) {
+          if (entry.duration >= 200) recent.push(entry.startTime);
+        }
+        while (recent.length && now - recent[0] > 30000) recent.shift();
+        if (recent.length >= 3) {
+          console.warn(`[CRASH-DIAG] WebGL long-task threshold (${recent.length} stalls/30s) — falling back to canvas renderer`);
+          _crashDiag.log(`WEBGL_FALLBACK: ${recent.length}`);
+          this._disableWebGLSticky('long-tasks');
+          this._webglAddon?.dispose();
+          this._webglAddon = null;
+          try { this._webglLongTaskObserver.disconnect(); } catch {}
+          this._webglLongTaskObserver = null;
+          try { this.terminal.refresh(0, this.terminal.rows - 1); } catch {}
+        }
+      });
+      this._webglLongTaskObserver.observe({ type: 'longtask', buffered: false });
+    } catch { /* longtask not supported */ }
+  }
+
+  _disableWebGLSticky(reason) {
+    try {
+      localStorage.setItem('codeman-webgl-disabled', JSON.stringify({ reason, at: Date.now() }));
+    } catch {}
+  }
+
   // ═══════════════════════════════════════════════════════════════
   // Event Listeners (Keyboard Shortcuts, Resize, Beforeunload)
   // ═══════════════════════════════════════════════════════════════