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/chatwidget/tests.rs

@@ -852,6 +852,7 @@ async fn make_chatwidget_manual(
         rate_limit_warnings: RateLimitWarningState::default(),
         rate_limit_switch_prompt: RateLimitSwitchPromptState::default(),
         rate_limit_poller: None,
+        adaptive_chunking: crate::streaming::chunking::AdaptiveChunkingPolicy::default(),
         stream_controller: None,
         plan_stream_controller: None,
         running_commands: HashMap::new(),