Add synchronized output support (DEC mode 2026)

Implement synchronized output mode (CSI ? 2026 h/l) which allows
applications to batch terminal updates and render them atomically,
preventing screen tearing during rapid output.

Features:
- BSU (CSI ? 2026 h) pauses rendering, buffering row updates
- ESU (CSI ? 2026 l) flushes buffer and renders atomically
- Configurable timeout via synchronizedOutputTimeout option (default 5s)
- Exposed via terminal.modes.synchronizedOutputMode

Closes #3375

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/browser/public/Terminal.ts

@@ -123,6 +123,7 @@ export class Terminal extends Disposable implements ITerminalApi {
       originMode: m.origin,
       reverseWraparoundMode: m.reverseWraparound,
       sendFocusMode: m.sendFocus,
+      synchronizedOutputMode: m.synchronizedOutput,
       wraparoundMode: m.wraparound
     };
   }