Move TUI on top of app server (parallel code) (#14717)

This PR replicates the `tui` code directory and creates a temporary
parallel `tui_app_server` directory. It also implements a new feature
flag `tui_app_server` to select between the two tui implementations.

Once the new app-server-based TUI is stabilized, we'll delete the old
`tui` directory and feature flag.

codex-rs/core/src/features.rs

@@ -180,6 +180,8 @@ pub enum Feature {
     VoiceTranscription,
     /// Enable experimental realtime voice conversation mode in the TUI.
     RealtimeConversation,
+    /// Route interactive startup to the app-server-backed TUI implementation.
+    TuiAppServer,
     /// Prevent idle system sleep while a turn is actively running.
     PreventIdleSleep,
     /// Use the Responses API WebSocket transport for OpenAI by default.
@@ -827,6 +829,16 @@ pub const FEATURES: &[FeatureSpec] = &[
         stage: Stage::UnderDevelopment,
         default_enabled: false,
     },
+    FeatureSpec {
+        id: Feature::TuiAppServer,
+        key: "tui_app_server",
+        stage: Stage::Experimental {
+            name: "App-server TUI",
+            menu_description: "Use the app-server-backed TUI implementation.",
+            announcement: "",
+        },
+        default_enabled: false,
+    },
     FeatureSpec {
         id: Feature::PreventIdleSleep,
         key: "prevent_idle_sleep",