Add an experimental UI tree renderer for the status line (#57970)

## What changed

- Add the disabled-by-default `tui_extensions_statusline` feature to render configured status items and their picker preview through a shared UI tree renderer.
- Support text, HTTP(S) links, rows, spacers, dividers, and gaps with rendering budgets, text sanitization, and width-aware truncation that keeps the ellipsis outside hyperlinks.
- Use plain text for the extension status line and hide the picker's theme color toggle while preserving the saved color preference.

## Testing

Add coverage for tree serialization and URL validation, Unicode widths, hyperlink clipping, picker previews, color preference preservation, and token usage updates across renderer switches.

GitOrigin-RevId: a6b87c2b1096765d61296d73e142834fcd53ceee

codex-rs/features/src/lib.rs

@@ -123,6 +123,8 @@ pub enum Feature {
     DaemonAutoStart,
     /// Allow the TUI to discover and select among multiple conversation backends.
     TuiMultiBackend,
+    /// Render the configured status line through the experimental TUI Extensions renderer.
+    TuiExtensionsStatusLine,
     /// Enable negotiated zstd compression for app-server WebSocket and UDS connections.
     AppServerWebSocketCompression,
 
@@ -1098,6 +1100,12 @@ pub const FEATURES: &[FeatureSpec] = &[
         stage: Stage::UnderDevelopment,
         default_enabled: false,
     },
+    FeatureSpec {
+        id: Feature::TuiExtensionsStatusLine,
+        key: "tui_extensions_statusline",
+        stage: Stage::UnderDevelopment,
+        default_enabled: false,
+    },
     FeatureSpec {
         id: Feature::AppServerWebSocketCompression,
         key: "app_server_websocket_compression",