Implement Phase 17a Prop Roles UI/UX Specification and associated functionality

- Added UI/UX specification for prop roles and set down mechanics in phase 17a.
- Created `PropStageRoles` script to manage normalized behavior for props, including depth, motion, and collision settings.
- Introduced `StagePhysicsPolicy` for shared physics configurations related to props and actors.
- Developed comprehensive tests for prop roles, including default settings, legacy migration, serialization, and runtime behavior.
- Ensured proper handling of set down actions, including clearance and orientation preservation.

AGENTS.md

@@ -54,6 +54,19 @@ authoring counterpart to the stickman editor — for creating, editing, and savi
 vector prop assets. It is launched via **F6** on `res://scenes/editors/prop/prop_editor.tscn` and is **not**
 the configured `run/main_scene`.
 
+## Shared Stage inspector UI
+
+- For new **in-stage selected-object inspectors**, use the approved
+  [Stage panel UI system spec](docs/specs/stage_panel_ui_system.md) as the design and
+  implementation reference. The shared controls live in `scripts/shared/stage_ui/`:
+  `StageInspectorPanel`, `StagePanelChoiceGroup`, `StagePanelToggleRow`, and
+  `StagePanelSwitchVisual`; shared visual tokens live under `stage_panels` in
+  `SandboxTheme` (`scripts/stage/sandbox_theme.gd` and `sandbox_theme.json`). Extend these
+  components and tokens before creating parallel panel styling or behavior.
+- 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.
+
 ## Required addon
 
 - **Scalable Vector Shapes 2D** (v2.27.7) at `addons/curved_lines_2d/`