fix(client): multi-primitive yield for write pacing (#85)

When the data-pacing path (chunkedTerminalWrite + deferred path of
flushPendingWrites) schedules its next chunk via requestAnimationFrame
alone, terminal output stalls indefinitely if rAF is starved. Three
real-world scenarios reproduce this in Chromium:

1. Window is occluded (fully covered by another window, or on a
   monitor that has gone to sleep). rAF drops to ~0Hz.
2. Tab is idle-throttled (no user interaction for ~5 min). Chromium
   intensive-throttling clamps setTimeout to 1Hz too.
3. Tab is in a background window. Both rAF and setTimeout slow to a
   crawl.

Replace the rAF-only scheduling with a _safeYield helper that races
three primitives in parallel:

- requestAnimationFrame (primary, fires at compositor rate).
- setTimeout(50) (fallback for visible-but-occluded windows).
- Worker postMessage tick (fallback for idle-throttled and
  background tabs; Workers are not subject to main-thread throttling
  — this is the React Scheduler trick).

The first one to fire wins via a `done` guard; the others become
no-ops. The Worker is built lazily on first call (4 lines of inline
JS via Blob URL); if Worker construction throws we silently fall
back to the other two primitives.

Replaces 6 requestAnimationFrame callsites that participate in data
pacing:
- 3 flushPendingWrites scheduling sites (live + deferred paths).
- 3 chunkedTerminalWrite sites (initial chunk, next-chunk loop,
  final finish-callback).

True animation use cases (scroll loop in scrollToBottom, fit-addon
reflow) stay on plain requestAnimationFrame — they are correctly
throttled when the user is not looking, by design.

File: src/web/public/terminal-ui.js (+70/-8).

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

src/web/public/terminal-ui.js

@@ -1151,7 +1151,7 @@ Object.assign(CodemanApp.prototype, {
 
     if (!this.writeFrameScheduled) {
       this.writeFrameScheduled = true;
-      requestAnimationFrame(() => {
+      this._safeYield(() => {
         // xterm.js 6.0 handles DEC 2026 sync markers natively — it buffers
         // content between 2026h/2026l and renders atomically. No need for
         // client-side incomplete-block detection; just flush every frame.
@@ -1176,7 +1176,7 @@ Object.assign(CodemanApp.prototype, {
     // Trigger a normal flush
     if (!this.writeFrameScheduled) {
       this.writeFrameScheduled = true;
-      requestAnimationFrame(() => {
+      this._safeYield(() => {
         this.flushPendingWrites();
         this.writeFrameScheduled = false;
       });
@@ -1264,7 +1264,7 @@ Object.assign(CodemanApp.prototype, {
       deferred = true;
       if (!this.writeFrameScheduled) {
         this.writeFrameScheduled = true;
-        requestAnimationFrame(() => {
+        this._safeYield(() => {
           this.flushPendingWrites();
           this.writeFrameScheduled = false;
         });
@@ -1336,9 +1336,70 @@ Object.assign(CodemanApp.prototype, {
     }
   },
 
+  /**
+   * Schedule cb via THREE racing primitives so data-pacing makes progress
+   * regardless of which scheduling primitive Chrome is throttling:
+   *   1. requestAnimationFrame — primary, fires at compositor rate
+   *      (may be 0Hz when window is occluded / on backgrounded monitor).
+   *   2. setTimeout(50) — fallback for occluded-but-visible windows
+   *      (clamped to 1Hz by Chrome's intensive wake-up throttling
+   *      after ~5 min of no user interaction).
+   *   3. Worker postMessage — bypasses intensive throttling entirely;
+   *      Workers are not subject to background-tab / idle-tab throttling
+   *      (the React Scheduler trick).
+   * Whichever fires first wins; the others are no-ops thanks to the
+   * `done` guard. Without all three, chunkedTerminalWrite and the deferred
+   * path of flushPendingWrites stall indefinitely when the substrate is
+   * degraded (visible-but-occluded window, OR idle-throttled tab, OR
+   * background tab on a different monitor).
+   */
+  _safeYield(cb) {
+    let done = false;
+    const wrapped = () => {
+      if (done) return;
+      done = true;
+      cb();
+    };
+    requestAnimationFrame(wrapped);
+    setTimeout(wrapped, 50);
+    this._workerYield(wrapped);
+  },
+
+  /**
+   * Lazy-init a tiny "tick" worker whose only job is to postMessage back to
+   * us as fast as possible, escaping main-thread throttling. The worker's
+   * setTimeout(0) is not subject to Chrome's intensive wake-up throttling
+   * even when the parent tab is idle.
+   */
+  _workerYield(cb) {
+    try {
+      if (this._yieldWorker === undefined) {
+        // First call: build the worker (or mark unavailable). Each
+        // postMessage in produces exactly one postMessage out — we count on
+        // FIFO 1:1 to drain queue entries.
+        const src = "onmessage=()=>setTimeout(()=>postMessage(0),0);";
+        const blob = new Blob([src], { type: 'application/javascript' });
+        const url = URL.createObjectURL(blob);
+        this._yieldWorker = new Worker(url);
+        URL.revokeObjectURL(url);
+        this._yieldQueue = [];
+        this._yieldWorker.onmessage = () => {
+          const fn = this._yieldQueue.shift();
+          if (fn) fn();
+        };
+      }
+      if (!this._yieldWorker) return;
+      this._yieldQueue.push(cb);
+      this._yieldWorker.postMessage(0);
+    } catch {
+      this._yieldWorker = null; // mark unavailable, future calls skip
+    }
+  },
+
   /**
    * Write large buffer to terminal in chunks to avoid UI jank.
-   * Uses requestAnimationFrame to spread work across frames.
+   * Uses _safeYield to spread work across frames; falls back to setTimeout
+   * and a tick-Worker so progress continues on occluded / idle-throttled tabs.
    * @param {string} buffer - The full terminal buffer to write
    * @param {number} chunkSize - Size of each chunk (default 128KB for smooth 60fps)
    * @returns {Promise<void>} - Resolves when all chunks written
@@ -1397,7 +1458,7 @@ Object.assign(CodemanApp.prototype, {
             `[CRASH-DIAG] chunkedTerminalWrite complete: ${cleanBuffer.length} bytes in ${_chunkCount} chunks, ${_totalMs.toFixed(0)}ms total`
           );
           // Wait one more frame for xterm to finish rendering before resolving
-          requestAnimationFrame(finish);
+          this._safeYield(finish);
           return;
         }
 
@@ -1412,12 +1473,13 @@ Object.assign(CodemanApp.prototype, {
           );
         offset += chunkSize;
 
-        // Schedule next chunk on next frame
-        requestAnimationFrame(writeChunk);
+        // Schedule next chunk; rAF if possible, else setTimeout/Worker
+        // fallback so progress doesn't stall on occluded/unfocused windows.
+        this._safeYield(writeChunk);
       };
 
       // Start writing
-      requestAnimationFrame(writeChunk);
+      this._safeYield(writeChunk);
     });
   },