Add Use Spot Overlay and Target Capture Bar functionality

- Introduced `StageUseTargetCaptureBar` class for managing user interactions with props and their use spots.
- Implemented UI elements including buttons for changing props and canceling actions.
- Added `PropUseSpotData` class for validation and serialization of prop use spots.
- Created tests for validating use spot data migration, serialization, and rule matching.
- Ensured proper handling of edge cases in use spot data, including unique IDs and finite positions.

AGENTS.md

@@ -66,6 +66,10 @@ the configured `run/main_scene`.
 - This kit currently targets in-stage selection/inspector cards. For modal dialogs, editor
   inspectors, or other distinct UI workflows, review the spec for reusable visual rules but
   decide whether the components' layout and interaction model fit before adopting them.
+- For temporary in-stage target or placement capture in **EDIT** or **DIRECT**, follow the
+  transient capture-bar pattern in the [Stage panel UI system spec](docs/specs/stage_panel_ui_system.md).
+  Keep the prompt and visible Cancel together, leave the mode pill unobstructed, and reuse or
+  generalize `StageUseTargetCaptureBar` instead of copying its styling.
 
 ## Required addon