feat: Integrate custom prop library and sandbox functionality

- Added support for custom props in the sandbox, including loading, saving, and rendering.
- Implemented `PropLibrary` for managing custom prop entries and scanning for `.prop` files.
- Introduced `PropVisuals` for shared visual and collision representation of custom props.
- Enhanced `StageSpawner` to handle spawning of custom props with layered visuals and collision hulls.
- Created `MissingPropPlaceholder` to manage missing custom props during scene reloads.
- Updated thumbnail generation to support custom props with unique keys based on file paths.
- Added tests for custom prop lifecycle, authored visual values, and missing prop handling.
- Refactored existing code to accommodate new prop functionalities and ensure backward compatibility.

scripts/asset_selector.gd

@@ -54,8 +54,9 @@ func _on_root_size_changed() -> void:
 func open(p_kind: String, entries: Array[Dictionary]) -> void:
 	kind = p_kind
 	_title_label.text = "Choose Your Stickman" if kind == "stickman" else "Choose a Prop"
-	_browse_button.visible = kind == "stickman"
-	_refresh_button.visible = kind == "stickman"
+	_browse_button.visible = kind == "stickman" or kind == "prop"
+	_refresh_button.visible = kind == "stickman" or kind == "prop"
+	_empty_label.text = "No props found. Browse for a .prop file." if kind == "prop" else "No stickmen found."
 	set_entries(entries)
 	popup_centered()