perf(tui2): cache transcript view rendering (#8693)

The transcript viewport draws every frame. Ratatui's Line::render_ref
does grapheme segmentation and span layout, so repeated redraws can burn
CPU during streaming even when the visible transcript hasn't changed.

Introduce TranscriptViewCache to reduce per-frame work:
- WrappedTranscriptCache memoizes flattened+wrapped transcript lines per
width, appends incrementally as new cells arrive, and rebuilds on width
change, truncation (backtrack), or transcript replacement.
- TranscriptRasterCache caches rasterized rows (Vec<Cell>) per line
index and user-row styling; redraws copy cells instead of rerendering
spans.

The caches are width-scoped and store base transcript content only;
selection highlighting and copy affordances are applied after drawing.
User rows include the row-wide base style in the cached raster.

Refactor transcript_render to expose append_wrapped_transcript_cell for
incremental building and add a test that incremental append matches the
full build.

Add docs/tui2/performance-testing.md as a playbook for macOS sample
profiles and hotspot greps.

Expand transcript_view_cache tests to cover rebuild conditions, raster
equivalence vs direct rendering, user-row caching, and eviction.

Test: cargo test -p codex-tui2

codex-rs/tui2/src/lib.rs

@@ -81,6 +81,7 @@ mod transcript_copy_ui;
 mod transcript_multi_click;
 mod transcript_render;
 mod transcript_selection;
+mod transcript_view_cache;
 mod tui;
 mod ui_consts;
 pub mod update_action;