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

docs/tui2/performance-testing.md

@@ -0,0 +1,97 @@
+# Performance testing (`codex-tui2`)
+
+This doc captures a repeatable workflow for investigating `codex-tui2` performance issues
+(especially high idle CPU and high CPU while streaming) and validating optimizations to the draw
+hot path.
+
+## Scope (this round)
+
+The current focus is the transcript draw hot path, specifically the cost of repeatedly rendering
+the same visible transcript lines via Ratatui’s `Line::render_ref` (notably grapheme segmentation
+and span layout).
+
+The intended mitigation is a **rasterization cache**: render a wrapped transcript `Line` into a
+row of `Cell`s once, cache it, and on subsequent redraws copy cached cells into the frame buffer.
+
+Key invariants:
+
+- The cache is width-scoped (invalidate on terminal width changes).
+- The cache stores **base content** only; selection highlight and copy affordances are applied
+  after rendering, so they don’t pollute cached rows.
+
+## Roles
+
+- Human: runs `codex-tui2` in an interactive terminal (e.g. Ghostty), triggers “idle” and
+  “streaming” scenarios, and captures profiles.
+- Assistant (or a script): reads profile output and extracts hotspots and deltas.
+
+## Baseline setup
+
+Build from a clean checkout:
+
+```sh
+cd codex-rs
+cargo build -p codex-tui2
+```
+
+Run `codex-tui2` in a terminal and get a PID (macOS):
+
+```sh
+pgrep -n codex-tui2
+```
+
+Track CPU quickly while reproducing:
+
+```sh
+top -pid "$(pgrep -n codex-tui2)"
+```
+
+## Capture profiles (macOS)
+
+Capture both an “idle” and a “streaming” profile so hotspots are not conflated:
+
+```sh
+sample "$(pgrep -n codex-tui2)" 1 -file /tmp/tui2.idle.sample.txt
+sample "$(pgrep -n codex-tui2)" 1 -file /tmp/tui2.streaming.sample.txt
+```
+
+For the streaming sample, trigger a response that emits many deltas (e.g. “Tell me a story”) so
+the stream runs long enough to sample.
+
+## Quick hotspot extraction
+
+These `rg` patterns keep the investigation grounded in the data:
+
+```sh
+# Buffer diff hot path (idle)
+rg -n "custom_terminal::diff_buffers|diff_buffers" /tmp/tui2.*.sample.txt | head -n 80
+
+# Transcript rendering hot path (streaming)
+rg -n "App::render_transcript_cells|Line::render|render_spans|styled_graphemes|GraphemeCursor::next_boundary" /tmp/tui2.*.sample.txt | head -n 120
+```
+
+## Rasterization-cache validation checklist
+
+After implementing a transcript rasterization cache, re-run the same scenarios and confirm:
+
+- Streaming sample shifts away from `unicode_segmentation::grapheme::GraphemeCursor::next_boundary`
+  stacks dominating the main thread.
+- CPU during streaming drops materially vs baseline for the same streaming load.
+- Idle CPU does not regress (redraw gating changes can mask rendering improvements; always measure
+  both idle and streaming).
+
+## Notes to record per run
+
+- Terminal size: width × height
+- Scenario: idle vs streaming (prompt + approximate response length)
+- CPU snapshot: `top` (directional)
+- Profile excerpt: 20–50 relevant lines for the dominant stacks
+
+## Code pointers
+
+- `codex-rs/tui2/src/transcript_view_cache.rs`: wrapped transcript memoization + per-line
+  rasterization cache (cached `Cell` rows).
+- `codex-rs/tui2/src/transcript_render.rs`: incremental helper used by the wrapped-line cache
+  (`append_wrapped_transcript_cell`).
+- `codex-rs/tui2/src/app.rs`: wiring in `App::render_transcript_cells` (uses cached rows instead of
+  calling `Line::render_ref` every frame).