feat: support synchronized output (DEC private mode 2026)

[smohekey]

What

Honour DEC private mode 2026 (synchronized output). Applications that redraw a full frame bracket each atomic update in ESC[?2026h … ESC[?2026l to tell the terminal "don't paint until I'm done." xterm.dart ignored the mode and painted every intermediate write, which shows as flicker during redraws — notably with ratatui-based TUIs such as the Codex CLI, which brackets every frame this way.

How

Suspend notifyListeners() between begin and end, then flush a single repaint on end so the completed frame appears atomically. A 150 ms safety timeout force-ends the update if the matching ?2026l is lost (e.g. across a reconnect), so the display can never stay frozen.

Reproduce

final terminal = Terminal()..resize(20, 5);
var repaints = 0;
terminal.addListener(() => repaints++);
terminal.write('\x1b[?2026h'); // begin
terminal.write('hello');       // held - no repaint
terminal.write('\x1b[?2026l'); // end - one repaint of the finished frame

Test

Adds test/src/synchronized_output_test.dart covering repaint suppression between begin/end, the flush on end, and that normal writes still repaint when no update is held.