perf(maestro): render terminals via WebGL

xterm's DOM renderer builds a node per cell, which is CPU-bound — the
wrong bottleneck on a phone, where an agent dumping output stutters
visibly on a Pixel 7 despite plenty of GPU headroom.

A WebGL context is scarce and revocable, so it follows the mounted view
rather than the connection: acquired in onMount, released in onUnmount.
ConnectionPool keeps MAX_PARKED connections alive behind the visible
ones, which would otherwise exceed the browser's context cap (~16 on
desktop Chrome, fewer on Android) and bring a parked terminal back
blank. Context loss falls back to the DOM renderer and retries against a
shared two-loss budget; a device that keeps dropping contexts stops
being asked. A device pixel ratio change rebuilds the glyph atlas, which
is otherwise left blurry.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

web/src/attach.ts

@@ -1,4 +1,5 @@
 import { FitAddon } from "@xterm/addon-fit";
+import { WebglAddon } from "@xterm/addon-webgl";
 import { Terminal } from "@xterm/xterm";
 import "@xterm/xterm/css/xterm.css";
 
@@ -10,6 +11,11 @@ import { applyModsToChar, cursorSeq, tildeSeq } from "./keys.js";
 import { requestToggleDrawer } from "./drawer.js";
 import { splitCommand, toolIconSvg } from "./toolIcon.js";
 import type { ClientControl, Uuid } from "./types.js";
+import { WebglPolicy } from "./webglRenderer.js";
+
+/** One budget for the whole page — see webglRenderer.ts for why contexts are
+ *  rationed rather than handed to every connection. */
+const webglPolicy = new WebglPolicy();
 
 /**
  * A single live attach: an xterm terminal bound to the daemon's attach WebSocket.
@@ -135,6 +141,72 @@ export function createAttachConnection(
   term.loadAddon(fit);
   term.open(body);
 
+  // GPU rendering is scoped to the mounted view, not the connection: acquired
+  // in onMount, released in onUnmount. See webglRenderer.ts.
+  let webgl: WebglAddon | null = null;
+  let dprMedia: MediaQueryList | null = null;
+
+  // The glyph atlas is rasterised for one device pixel ratio. Dragging the
+  // window to a display with a different one (or an Android browser-zoom
+  // change) leaves every cell blurry until the atlas is rebuilt.
+  const onDprChange = () => {
+    try {
+      term.clearTextureAtlas();
+    } catch {
+      // ignore — nothing to rebuild if the renderer already went away
+    }
+    watchDpr();
+  };
+
+  function watchDpr(): void {
+    dprMedia?.removeEventListener("change", onDprChange);
+    dprMedia = null;
+    try {
+      dprMedia = window.matchMedia?.(`(resolution: ${window.devicePixelRatio}dppx)`) ?? null;
+      dprMedia?.addEventListener("change", onDprChange);
+    } catch {
+      // ignore — a browser without `resolution` media queries just keeps the
+      // atlas it has, which is the pre-WebGL behaviour anyway
+    }
+  }
+
+  function releaseWebgl(): void {
+    dprMedia?.removeEventListener("change", onDprChange);
+    dprMedia = null;
+    if (!webgl) return;
+    const addon = webgl;
+    webgl = null;
+    try {
+      addon.dispose(); // xterm falls back to the DOM renderer
+    } catch {
+      // ignore — a lost context can throw on the way out
+    }
+  }
+
+  function acquireWebgl(): void {
+    if (webgl || disposed || !webglPolicy.enabled()) return;
+    try {
+      const addon = new WebglAddon();
+      // Context loss is routine on Android — backgrounding the tab is enough.
+      // Drop back to the DOM renderer, then retry only while the shared budget
+      // allows and this view is still the one on screen.
+      addon.onContextLoss(() => {
+        webglPolicy.noteContextLoss();
+        releaseWebgl();
+        if (mounted) acquireWebgl();
+      });
+      term.loadAddon(addon);
+      webgl = addon;
+      watchDpr();
+    } catch {
+      // Context creation failed outright (no GPU, or the browser is already at
+      // its context cap). Count it like a loss so we stop asking, and stay on
+      // the DOM renderer.
+      webglPolicy.noteContextLoss();
+      webgl = null;
+    }
+  }
+
   // Plain Escape hands the keyboard back to the session list instead of the
   // shell/app. `false` tells xterm to skip its own handling of this event.
   term.attachCustomKeyEventHandler((e) => {
@@ -402,6 +474,7 @@ export function createAttachConnection(
     el,
     onMount() {
       mounted = true;
+      acquireWebgl();
       // Defer a tick so the element has its content-pane dimensions before fit.
       requestAnimationFrame(() => {
         tryFit();
@@ -411,6 +484,9 @@ export function createAttachConnection(
     },
     onUnmount() {
       mounted = false;
+      // Parked views hand their GPU context back: the browser's cap is small
+      // (smaller still on Android) and MAX_PARKED connections outlive the pane.
+      releaseWebgl();
     },
     focusTerminal() {
       term.focus();
@@ -427,6 +503,7 @@ export function createAttachConnection(
       visual?.removeEventListener("resize", onResize);
       dataDisp.dispose();
       resizeDisp.dispose();
+      releaseWebgl();
       try {
         ws.close();
       } catch {