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/prop_block.gd

@@ -10,6 +10,8 @@ extends RigidBody2D
 ## properties update the children live. A material preset selector sets mass,
 ## friction/bounce (PhysicsMaterial), and themed visuals.
 
+const PropVisuals := preload("res://scripts/prop_visuals.gd")
+
 # ---------------------------------------------------------------------------
 # Enums
 # ---------------------------------------------------------------------------
@@ -91,6 +93,8 @@ var _outline: Line2D
 var _collision_polygon: CollisionPolygon2D
 var _collision_shape: CollisionShape2D
 var _circle_shape: CircleShape2D
+var _custom_shapes: bool = false
+var _visual_local_bounds := Rect2()
 
 # ---------------------------------------------------------------------------
 # Signals
@@ -241,10 +245,34 @@ func _apply_style() -> void:
 func _apply_material_preset() -> void:
 	mass = mass_for(material_preset)
 	physics_material_override = physics_material(material_preset)
-	if material_preset != MaterialPreset.NONE:
+	if not _custom_shapes and material_preset != MaterialPreset.NONE:
 		fill_color = tint_for(material_preset)
 		outline_color = tint_for(material_preset).darkened(0.55)
 
+
+func set_custom_shapes(shapes: Array[Dictionary], collision_hull: PackedVector2Array) -> void:
+	_ensure_children()
+	_custom_shapes = true
+	_polygon.visible = false
+	_outline.visible = false
+	_set_collision_shape_enabled(false)
+	_set_collision_polygon_enabled(true)
+	_collision_polygon.polygon = collision_hull
+	var visuals := get_node_or_null(NodePath("Visuals")) as Node2D
+	if visuals == null:
+		visuals = Node2D.new(); visuals.name = "Visuals"; add_child(visuals)
+	for child: Node in visuals.get_children(): child.queue_free()
+	_visual_local_bounds = PropVisuals.build(visuals, shapes)
+	_apply_material_preset()
+
+
+func has_custom_shapes() -> bool:
+	return _custom_shapes
+
+
+func get_visual_local_bounds() -> Rect2:
+	return _visual_local_bounds
+
 # ---------------------------------------------------------------------------
 # Static preset factories
 # ---------------------------------------------------------------------------