perf(tui): cap redraw scheduling to 60fps (#8499)

Clamp frame draw notifications in the `FrameRequester` scheduler so we
don't redraw more frequently than a user can perceive.

This applies to both `codex-tui` and `codex-tui2`, and keeps the
draw/dispatch loops simple by centralizing the rate limiting in a small
helper module.

- Add `FrameRateLimiter` (pure, unit-tested) to clamp draw deadlines
- Apply the limiter in the scheduler before emitting `TuiEvent::Draw`
- Use immediate redraw requests for scroll paths (scheduler now
coalesces + clamps)
- Add scheduler tests covering immediate/delayed interactions

codex-rs/tui2/src/tui.rs

@@ -47,6 +47,7 @@ use crate::tui::job_control::SUSPEND_KEY;
 use crate::tui::job_control::SuspendContext;
 
 mod alt_screen_nesting;
+mod frame_rate_limiter;
 mod frame_requester;
 #[cfg(unix)]
 mod job_control;