feat(tui): retire the tui2 experiment (#9640)

## Summary
- Retire the experimental TUI2 implementation and its feature flag.
- Remove TUI2-only config/schema/docs so the CLI stays on the
terminal-native path.
- Keep docs aligned with the legacy TUI while we focus on redraw-based
improvements.

## Customer impact
- Retires the TUI2 experiment and keeps Codex on the proven
terminal-native UI while we invest in redraw-based improvements to the
existing experience.

## Migration / compatibility
- If you previously set tui2-related options in config.toml, they are
now ignored and Codex continues using the existing terminal-native TUI
(no action required).

## Context
- What worked: a transcript-owned viewport delivered excellent resize
rewrap and high-fidelity copy (especially for code).
- Why stop: making that experience feel fully native across the
environment matrix (terminal emulator, OS, input modality, multiplexer,
font/theme, alt-screen behavior) creates a combinatorial explosion of
edge cases.
- What next: we are focusing on redraw-based improvements to the
existing terminal-native TUI so scrolling, selection, and copy remain
native while resize/redraw correctness improves.

## Testing
- just write-config-schema
- just fmt
- cargo clippy --fix --all-features --tests --allow-dirty --allow-no-vcs
-p codex-core
- cargo clippy --fix --all-features --tests --allow-dirty --allow-no-vcs
-p codex-cli
- cargo check
- cargo test -p codex-core
- cargo test -p codex-cli

docs/exit-confirmation-prompt-design.md

@@ -1,8 +1,8 @@
-# Exit and shutdown flow (tui + tui2)
+# Exit and shutdown flow (tui)
 
-This document describes how exit, shutdown, and interruption work in the Rust TUIs (`codex-rs/tui`
-and `codex-rs/tui2`). It is intended for Codex developers and Codex itself when reasoning about
-future exit/shutdown changes.
+This document describes how exit, shutdown, and interruption work in the Rust TUI (`codex-rs/tui`).
+It is intended for Codex developers and Codex itself when reasoning about future exit/shutdown
+changes.
 
 This doc replaces earlier separate history and design notes. High-level history is summarized
 below; full details are captured in PR #8936.

docs/tui-chat-composer.md

@@ -6,12 +6,10 @@ for Windows terminals.
 Primary implementations:
 
 - `codex-rs/tui/src/bottom_pane/chat_composer.rs`
-- `codex-rs/tui2/src/bottom_pane/chat_composer.rs`
 
 Paste-burst detector:
 
 - `codex-rs/tui/src/bottom_pane/paste_burst.rs`
-- `codex-rs/tui2/src/bottom_pane/paste_burst.rs`
 
 ## What problem is being solved?
 
@@ -222,12 +220,6 @@ The `PasteBurst` logic is currently exercised through `ChatComposer` integration
   - `question_mark_does_not_toggle_during_paste_burst`
   - `burst_paste_fast_small_buffers_and_flushes_on_stop`
   - `burst_paste_fast_large_inserts_placeholder_on_flush`
-- `codex-rs/tui2/src/bottom_pane/chat_composer.rs`
-  - `non_ascii_burst_handles_newline`
-  - `ascii_burst_treats_enter_as_newline`
-  - `question_mark_does_not_toggle_during_paste_burst`
-  - `burst_paste_fast_small_buffers_and_flushes_on_stop`
-  - `burst_paste_fast_large_inserts_placeholder_on_flush`
 
 This document calls out some additional contracts (like “flush before clearing”) that are not yet
 fully pinned by dedicated `PasteBurst` unit tests.

docs/tui2/performance-testing.md

@@ -1,97 +0,0 @@
-# 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).