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/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