Implement interactive prop handling for StickmanRig

- Added PropTrajectory class for deterministic ballistic calculations.
- Introduced StickmanPropInteraction component to manage prop interactions (pickup, drop, throw).
- Implemented grip resolution and attachment logic for props.
- Added signals for interaction events (prop_picked_up, prop_dropped, prop_thrown).
- Defined specifications for prop interaction lifecycle and event rules.
- Ensured compatibility with existing prop systems and added new capabilities for prop pickup.

scripts/prop_block.gd

@@ -11,6 +11,7 @@ extends RigidBody2D
 ## friction/bounce (PhysicsMaterial), and themed visuals.
 
 const PropVisuals := preload("res://scripts/prop_visuals.gd")
+const PropAnchorData := preload("res://scripts/prop_anchor_data.gd")
 
 # ---------------------------------------------------------------------------
 # Enums
@@ -22,6 +23,11 @@ enum MaterialPreset { NONE, WOOD, RUBBER, CARDBOARD, METAL }
 
 ## Stable library provenance used by Sandbox Stage scene files.
 var spawn_id: String = ""
+## Runtime interaction metadata. Both values are additive so existing scenes
+## and built-in templates remain pickup-enabled.
+var can_be_picked_up: bool = true
+var anchors: Dictionary = PropAnchorData.default_anchors()
+var held_by_instance_id: int = 0
 
 # ---------------------------------------------------------------------------
 # Child node names
@@ -275,7 +281,16 @@ func has_custom_shapes() -> bool:
 
 
 func get_visual_local_bounds() -> Rect2:
-	return _visual_local_bounds
+	if _custom_shapes:
+		return _visual_local_bounds
+	if shape_type == ShapeType.CIRCLE:
+		return Rect2(Vector2.ONE * -radius, Vector2.ONE * radius * 2.0)
+	if not polygon_points.is_empty():
+		var bounds: Rect2 = Rect2(polygon_points[0], Vector2.ZERO)
+		for point: Vector2 in polygon_points:
+			bounds = bounds.expand(point)
+		return bounds
+	return Rect2()
 
 ## RigidBody2D transforms do not reliably retain scale during simulation. Move
 ## the authored uniform scale into child visuals and collision geometry for