Add selection and copying to the transcript viewer (#46732)

## What changed

- Add mouse selection, word and line selection, drag autoscrolling, and keyboard selection with `Ctrl+Space`. Support copying selected text and opening links with a modified click.
- Anchor reading positions and selections to source text so resizing, streaming completion, and history pagination preserve the displayed content. Preserve hard line breaks, indentation, tabs, and graphemes when copying, while excluding display gutters and soft wraps.
- Rejoin activity groups split across history pages while retaining the reader's displayed revision.
- Distinguish confirmed clipboard writes from unconfirmed terminal requests, and retain native clipboard ownership after the transcript overlay closes.
- Manage mouse capture and keyboard modes across overlay transitions, editor handoffs, and suspend/resume.

## Testing

Add regression tests for selection during live updates and pagination, source-text copying across resize, clipboard confirmation and lifetime, and terminal mode restoration.

GitOrigin-RevId: 6b2438f576567cbba755b2405915a25acf009b8d

codex-rs/tui/src/thread_transcript.rs

@@ -30,35 +30,15 @@ mod exploration_groups;
 mod other_items;
 pub(crate) mod tools;
 
-#[allow(
-    unused_imports,
-    reason = "Used by later layers of the TUI refresh stack."
-)]
 pub(crate) use activity_pages::fold_trailing_activity_details;
-#[allow(
-    unused_imports,
-    reason = "Used by later layers of the TUI refresh stack."
-)]
 pub(crate) use activity_pages::is_hidden_activity_detail;
 #[allow(
     unused_imports,
     reason = "Used by later layers of the TUI refresh stack."
 )]
 pub(crate) use computer_groups::join_computer_groups;
-#[allow(
-    unused_imports,
-    reason = "Used by later layers of the TUI refresh stack."
-)]
 pub(crate) use computer_groups::older_computer_group;
-#[allow(
-    unused_imports,
-    reason = "Used by later layers of the TUI refresh stack."
-)]
 pub(crate) use exploration_groups::join_exploration_groups;
-#[allow(
-    unused_imports,
-    reason = "Used by later layers of the TUI refresh stack."
-)]
 pub(crate) use exploration_groups::older_exploration_group;
 
 pub(crate) type TranscriptCells = Vec<Arc<dyn HistoryCell>>;