use a central animation loop (#2268)

instead of each shimmer needing to have its own animation thread, have
render_ref schedule a new frame if it wants one and coalesce to the
earliest next frame. this also makes the animations
frame-timing-independent, based on start time instead of frame count.

codex-rs/tui/src/app_event.rs

@@ -2,6 +2,7 @@ use codex_core::protocol::Event;
 use codex_file_search::FileMatch;
 use crossterm::event::KeyEvent;
 use ratatui::text::Line;
+use std::time::Duration;
 
 use crate::app::ChatWidgetArgs;
 use crate::slash_command::SlashCommand;
@@ -17,6 +18,10 @@ pub(crate) enum AppEvent {
     /// Actually draw the next frame.
     Redraw,
 
+    /// Schedule a one-shot animation frame roughly after the given duration.
+    /// Multiple requests are coalesced by the central frame scheduler.
+    ScheduleFrameIn(Duration),
+
     KeyEvent(KeyEvent),
 
     /// Text pasted from the terminal clipboard.