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/common/TestUtils.test.ts

@@ -102,6 +102,7 @@ export class MockCoreService implements ICoreService {
     origin: false,
     reverseWraparound: false,
     sendFocus: false,
+    synchronizedOutput: false,
     wraparound: true
   };
   public onData: Event<string> = new Emitter<string>().event;