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/config.schema.json

@@ -4150,6 +4150,11 @@
           "default": false,
           "description": "Start the composer in Vim mode (`Normal`) by default. Defaults to `false`.",
           "type": "boolean"
+        },
+        "whimsy": {
+          "default": true,
+          "description": "Enable decorative effects such as Astra composer stars. Also requires animations. Defaults to `true`.",
+          "type": "boolean"
         }
       },
       "type": "object"