feat: experimental menu (#8071)

This will automatically render any `Stage::Beta` features.

The change only gets applied to the *next session*. This started as a
bug but actually this is a good thing to prevent out of distribution
push

<img width="986" height="288" alt="Screenshot 2025-12-15 at 15 38 35"
src="https://github.com/user-attachments/assets/78b7a71d-0e43-4828-a118-91c5237909c7"
/>


<img width="509" height="109" alt="Screenshot 2025-12-15 at 17 35 44"
src="https://github.com/user-attachments/assets/6933de52-9b66-4abf-b58b-a5f26d5747e2"
/>

codex-rs/tui/src/bottom_pane/selection_popup_common.rs

@@ -15,6 +15,7 @@ use crate::key_hint::KeyBinding;
 use super::scroll_state::ScrollState;
 
 /// A generic representation of a display row for selection popups.
+#[derive(Default)]
 pub(crate) struct GenericDisplayRow {
     pub name: String,
     pub display_shortcut: Option<KeyBinding>,