feat(tui): pace catch-up stream chunking with hysteresis (#10461)

## Summary
- preserve baseline streaming behavior (smooth mode still commits one
line per 50ms tick)
- extract adaptive chunking policy and commit-tick orchestration from
ChatWidget into `streaming/chunking.rs` and `streaming/commit_tick.rs`
- add hysteresis-based catch-up behavior with bounded batch draining to
reduce queue lag without bursty single-frame jumps
- document policy behavior, tuning guidance, and debug flow in rustdoc +
docs

## Testing
- just fmt
- cargo test -p codex-tui

codex-rs/tui/src/pager_overlay.rs

@@ -17,7 +17,6 @@
 
 use std::io::Result;
 use std::sync::Arc;
-use std::time::Duration;
 
 use crate::chatwidget::ActiveCellTranscriptKey;
 use crate::history_cell::HistoryCell;
@@ -299,7 +298,7 @@ impl PagerView {
             }
         }
         tui.frame_requester()
-            .schedule_frame_in(Duration::from_millis(16));
+            .schedule_frame_in(crate::tui::TARGET_FRAME_INTERVAL);
         Ok(())
     }