Add prop interaction system and throw mechanics specification

- Introduced `stickman_prop_interactor.gd` to manage prop interactions.
- Created `phase_11_prop_interactions.md` detailing the design and implementation of prop pickup, drop, and throw mechanics.
- Defined interaction states, action lifecycles, and event triggers for prop handling.
- Specified constraints for throw actions, including target positioning and velocity calculations.
- Established acceptance criteria for prop interactions and lifecycle management.

scripts/stage_spawner.gd

@@ -176,6 +176,8 @@ func spawn_prop_asset(asset_id: String, object_position: Vector2) -> PropBlock:
 	else:
 		prop = PROP_UTILS.spawn_prop(_world, object_position, template["payload"], int(template["material_preset"]), Vector2.ZERO)
 	prop.spawn_id = String(template.get("id", asset_id))
+	prop.set_interaction_anchors(template.get("anchors", {}) as Dictionary)
+	prop.ensure_default_grip()
 	return prop