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

[aakhter]

Summary

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.

Behavior

• Threshold: 3 long tasks of ≥ 200ms each within a 30s rolling window.
• 5s grace period after init — skips noisy initial-load stalls so a slow first paint doesn't trip the guard.
• On trigger: dispose the WebGL addon, write a sticky disable to localStorage (codeman-webgl-disabled) with { reason, at }, and terminal.refresh() 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.
• ?webgl=force clears the sticky entry and forces WebGL to re-init.
• ?nowebgl unchanged — still wins as the manual escape hatch.
• 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; two new helpers: _installWebGLLongTaskGuard() and _disableWebGLSticky(reason).
• src/web/public/terminal-ui.js — WebGL init reads the sticky entry with 7-day expiry, honors ?webgl=force, threads sticky into skipWebGL alongside the existing mobile + ?nowebgl gates.

Browser support

PerformanceObserver longtask is supported in Chromium and Edge. The try/catch around .observe({ type: 'longtask' }) makes Firefox/Safari (no longtask entry type) silently no-op — they just keep WebGL with no watchdog.

Test plan

• Normal load on Chrome desktop: WebGL initializes, watchdog installs, no false trips after 30s of normal use.
• Force a long task during typing (e.g. for (let i=0;i<3;i++) { const t=performance.now(); while(performance.now()-t<250){} await new Promise(r=>setTimeout(r,1000)); } in DevTools console): after 3 stalls within 30s, the canvas renderer takes over silently and localStorage.codeman-webgl-disabled is set.
• Reload: WebGL stays off, console logs the sticky-disable message.
• ?webgl=force: sticky entry cleared, WebGL re-initializes.
• After 7 days (or manually setting at to an old timestamp): sticky entry auto-clears on next load.
• Firefox/Safari: WebGL stays on (no longtask entry type — guard silently no-ops).

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