tui2: stop baking streaming wraps; reflow agent markdown (#8761)

Background
Streaming assistant prose in tui2 was being rendered with viewport-width
wrapping during streaming, then stored in history cells as already split
`Line`s. Those width-derived breaks became indistinguishable from hard
newlines, so the transcript could not "un-split" on resize. This also
degraded copy/paste, since soft wraps looked like hard breaks.

What changed
- Introduce width-agnostic `MarkdownLogicalLine` output in
`tui2/src/markdown_render.rs`, preserving markdown wrap semantics:
initial/subsequent indents, per-line style, and a preformatted flag.
- Update the streaming collector (`tui2/src/markdown_stream.rs`) to emit
logical lines (newline-gated) and remove any captured viewport width.
- Update streaming orchestration (`tui2/src/streaming/*`) to queue and
emit logical lines, producing `AgentMessageCell::new_logical(...)`.
- Make `AgentMessageCell` store logical lines and wrap at render time in
`HistoryCell::transcript_lines_with_joiners(width)`, emitting joiners so
copy/paste can join soft-wrap continuations correctly.

Overlay deferral
When an overlay is active, defer *cells* (not rendered `Vec<Line>`) and
render them at overlay close time. This avoids baking width-derived
wraps based on a stale width.

Tests + docs
- Add resize/reflow regression tests + snapshots for streamed agent
output.
- Expand module/API docs for the new logical-line streaming pipeline and
clarify joiner semantics.
- Align scrollback-related docs/comments with current tui2 behavior
(main draw loop does not flush queued "history lines" to the terminal).

More details
See `codex-rs/tui2/docs/streaming_wrapping_design.md` for the full
problem statement and solution approach, and
`codex-rs/tui2/docs/tui_viewport_and_history.md` for viewport vs printed
output behavior.

codex-rs/tui2/src/chatwidget.rs

@@ -1023,9 +1023,9 @@ impl ChatWidget {
                 self.needs_final_message_separator = false;
                 needs_redraw = true;
             }
-            self.stream_controller = Some(StreamController::new(
-                self.last_rendered_width.get().map(|w| w.saturating_sub(2)),
-            ));
+            // Streaming must not capture the current viewport width: width-derived wraps are
+            // applied later, at render time, so the transcript can reflow on resize.
+            self.stream_controller = Some(StreamController::new());
         }
         if let Some(controller) = self.stream_controller.as_mut()
             && controller.push(&delta)