Remove the legacy TUI split (#15922)

This is the part 1 of 2 PRs that will delete the `tui` /
`tui_app_server` split. This part simply deletes the existing `tui`
directory and marks the `tui_app_server` feature flag as removed. I left
the `tui_app_server` feature flag in place for now so its presence
doesn't result in an error. It is simply ignored.

Part 2 will rename the `tui_app_server` directory `tui`. I did this as
two parts to reduce visible code churn.

docs/tui-alternate-screen.md

@@ -75,7 +75,7 @@ let terminal_info = codex_core::terminal::terminal_info();
 !matches!(terminal_info.multiplexer, Some(Multiplexer::Zellij { .. }))
 ```
 
-This detection happens in the helper function `determine_alt_screen_mode()` in `codex-rs/tui/src/lib.rs`.
+This detection happens in the helper function `determine_alt_screen_mode()` in `codex-rs/tui_app_server/src/lib.rs`.
 
 ### Configuration Schema
 

docs/tui-chat-composer.md

@@ -5,11 +5,11 @@ for Windows terminals.
 
 Primary implementations:
 
-- `codex-rs/tui/src/bottom_pane/chat_composer.rs`
+- `codex-rs/tui_app_server/src/bottom_pane/chat_composer.rs`
 
 Paste-burst detector:
 
-- `codex-rs/tui/src/bottom_pane/paste_burst.rs`
+- `codex-rs/tui_app_server/src/bottom_pane/paste_burst.rs`
 
 ## What problem is being solved?
 
@@ -68,7 +68,7 @@ while still providing a richer recall experience for in-session edits.
 ## Config gating for reuse
 
 `ChatComposer` now supports feature gating via `ChatComposerConfig`
-(`codex-rs/tui/src/bottom_pane/chat_composer.rs`). The default config preserves current chat
+(`codex-rs/tui_app_server/src/bottom_pane/chat_composer.rs`). The default config preserves current chat
 behavior.
 
 Flags:
@@ -90,7 +90,7 @@ Key effects when disabled:
   dropping the draft.
 
 Built-in slash command availability is centralized in
-`codex-rs/tui/src/bottom_pane/slash_commands.rs` and reused by both the composer and the command
+`codex-rs/tui_app_server/src/bottom_pane/slash_commands.rs` and reused by both the composer and the command
 popup so gating stays in sync.
 
 ## Submission flow (Enter/Tab)
@@ -345,7 +345,7 @@ Non-char input must not leak burst state across unrelated actions:
 
 The `PasteBurst` logic is currently exercised through `ChatComposer` integration tests.
 
-- `codex-rs/tui/src/bottom_pane/chat_composer.rs`
+- `codex-rs/tui_app_server/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`

docs/tui-stream-chunking-review.md

@@ -25,13 +25,13 @@ and visible output lags behind received output.
 
 ## Where the logic lives
 
-- `codex-rs/tui/src/streaming/chunking.rs`
+- `codex-rs/tui_app_server/src/streaming/chunking.rs`
   - Adaptive policy, mode transitions, and drain-plan selection.
-- `codex-rs/tui/src/streaming/commit_tick.rs`
+- `codex-rs/tui_app_server/src/streaming/commit_tick.rs`
   - Orchestration for each commit tick: snapshot, decide, drain, trace.
-- `codex-rs/tui/src/streaming/controller.rs`
+- `codex-rs/tui_app_server/src/streaming/controller.rs`
   - Queue/drain primitives used by commit-tick orchestration.
-- `codex-rs/tui/src/chatwidget.rs`
+- `codex-rs/tui_app_server/src/chatwidget.rs`
   - Integration point that invokes commit-tick orchestration and handles UI
     lifecycle events.
 

docs/tui-stream-chunking-tuning.md

@@ -6,8 +6,8 @@ changing the underlying policy shape.
 ## Scope
 
 Use this guide when adjusting queue-pressure thresholds and hysteresis windows in
-`codex-rs/tui/src/streaming/chunking.rs`, and baseline commit cadence in
-`codex-rs/tui/src/app.rs`.
+`codex-rs/tui_app_server/src/streaming/chunking.rs`, and baseline commit cadence in
+`codex-rs/tui_app_server/src/app.rs`.
 
 This guide is about tuning behavior, not redesigning the policy.