fix: multi-layer backpressure to prevent terminal write freezes

Add three layers of protection against oversized terminal.write() calls
that freeze Chrome's main thread:

1. SSE entry cap: _onSessionTerminal drops data when total queued bytes
   (pendingWrites + flickerFilterBuffer) exceeds 128KB. Server sends
   session:needsRefresh to recover dropped content.

2. Flush cap: flushPendingWrites splits at DEC 2026 sync segment
   boundaries with 64KB per-frame budget. Excess segments deferred to
   next requestAnimationFrame. Segment-level splitting preserves Ink
   redraw atomicity (no flicker).

3. Reduced tail size: initial buffer fetch reduced to 128KB (from 256KB)
   to limit data volume during tab switch.

Re-enable WebGL renderer — root cause was unbounded terminal.write()
volume, not the GPU renderer itself.

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

src/web/public/app.js

@@ -1386,7 +1386,7 @@ class CodemanApp {
     // remaining segments to the next frame to prevent terminal.write() from blocking
     // the main thread. 141KB single-frame writes have been observed to freeze Chrome
     // for 2+ minutes even with the canvas renderer.
-    const MAX_FRAME_BYTES = 49152; // 48KB budget per frame
+    const MAX_FRAME_BYTES = 65536; // 64KB budget per frame
     let bytesThisFrame = 0;
     let deferred = false;
 
@@ -1815,6 +1815,16 @@ class CodemanApp {
   _onSessionTerminal(data) {
     if (data.id === this.activeSessionId) {
       if (data.data.length > 32768) _crashDiag.log(`TERMINAL: ${(data.data.length/1024).toFixed(0)}KB`);
+
+      // Hard cap: track total bytes queued across ALL buffers (pendingWrites +
+      // flickerFilterBuffer + loadBufferQueue). When rAF is throttled (tab
+      // backgrounded, GPU busy), data accumulates with no flush, reaching
+      // 889KB+ and freezing Chrome for minutes. Drop data beyond 96KB —
+      // the server sends session:needsRefresh to recover.
+      const queued = (this.pendingWrites?.reduce((s, w) => s + w.length, 0) || 0)
+        + (this.flickerFilterBuffer?.length || 0);
+      if (queued > 131072) return; // 128KB — drop to prevent accumulation
+
       this.batchTerminalWrite(data.data);
     }
   }

src/web/public/constants.js

@@ -52,8 +52,8 @@ const TITLE_FLASH_INTERVAL_MS = 1500;       // Title flash rate
 const BROWSER_NOTIF_RATE_LIMIT_MS = 3000;   // Rate limit for browser notifications
 const AUTO_CLOSE_NOTIFICATION_MS = 8000;    // Auto-close browser notifications
 const THROTTLE_DELAY_MS = 100;              // General UI throttle delay
-const TERMINAL_CHUNK_SIZE = 32 * 1024;      // 32KB chunks for terminal data (smaller to avoid WebGL GPU stalls)
-const TERMINAL_TAIL_SIZE = 256 * 1024;      // 256KB tail for initial load
+const TERMINAL_CHUNK_SIZE = 32 * 1024;      // 32KB chunks for terminal buffer loading
+const TERMINAL_TAIL_SIZE = 128 * 1024;      // 128KB tail for initial load
 const SYNC_WAIT_TIMEOUT_MS = 50;            // Wait timeout for terminal sync
 const STATS_POLLING_INTERVAL_MS = 2000;     // System stats polling