Add Astra sparkle effects to the TUI composer (#42842)

## What changed

- Render sparse, fading stars across the composer when using an Astra model in a true-color terminal.
- Preserve composer content, cursor state, effort effects, and terminal colors, and pause the animation while popups are open.
- Add the default-on `tui.whimsy` setting so decorative effects can be disabled independently. Sparkles also respect `tui.animations`.

## Testing

- Cover model and setting eligibility, terminal color handling, protected composer cells, popup behavior, effort effects, and layout stability.

GitOrigin-RevId: fe0471ebef0bca21c44e1d8f731d46959eba0165

codex-rs/core/src/config/config_tests.rs

@@ -1256,6 +1256,7 @@ fn config_toml_deserializes_model_availability_nux() {
         Tui {
             notification_settings: TuiNotificationSettings::default(),
             animations: true,
+            whimsy: true,
             show_tooltips: true,
             auto_recap: true,
             disable_paste_burst: None,
@@ -4269,6 +4270,7 @@ fn tui_config_missing_notifications_field_defaults_to_enabled() {
         Tui {
             notification_settings: TuiNotificationSettings::default(),
             animations: true,
+            whimsy: true,
             show_tooltips: true,
             auto_recap: true,
             disable_paste_burst: None,

codex-rs/core/src/config/mod.rs

@@ -740,6 +740,9 @@ pub struct Config {
     /// Enable ASCII animations and shimmer effects in the TUI.
     pub animations: bool,
 
+    /// Enable decorative TUI effects such as Astra composer stars.
+    pub tui_whimsy: bool,
+
     /// Show startup tooltips in the TUI welcome screen.
     pub show_tooltips: bool,
 
@@ -4343,6 +4346,7 @@ impl Config {
                 .map(|t| t.notification_settings.clone())
                 .unwrap_or_default(),
             animations: cfg.tui.as_ref().map(|t| t.animations).unwrap_or(true),
+            tui_whimsy: cfg.tui.as_ref().map(|t| t.whimsy).unwrap_or(true),
             show_tooltips: cfg.tui.as_ref().map(|t| t.show_tooltips).unwrap_or(true),
             tui_auto_recap: cfg.tui.as_ref().map(|t| t.auto_recap).unwrap_or(/*default*/ true),
             model_availability_nux: cfg