Add sandbox stage builder scripts and functionality

- Introduced StageGizmos for hover highlighting, selection outlines, and rotation handles in the sandbox stage builder.
- Added StageGrid for an optional world-space grid overlay that adjusts with camera panning and zooming.
- Implemented StageSelection for geometric hit-testing and selection management of nodes in the sandbox.
- Created StageSpawner as a registry-driven factory for spawning terrain, props, and stickmen, allowing for dynamic template management.
- Each script includes necessary constants, state management, and public API methods for interaction.

scripts/stickman_rig.gd

@@ -436,6 +436,35 @@ func request_recovery() -> void:
 		_start_recovery()
 
 
+## Instantly snaps the rig back to its authored standing pose — no stand-up
+## tween. Used by the sandbox stage so a stickman "reappears" at its starting
+## position/state on return to EDIT (instead of animating the recovery glide).
+func snap_to_standing() -> void:
+	if state == RigState.ANIMATED:
+		return
+	if state == RigState.RAGDOLL:
+		_destroy_ragdoll()
+	else:
+		_cancel_recovery()
+	# Set the IK targets directly to the standing pose (no tween).
+	for marker_name: String in STAND_POSE:
+		var marker := _get_ik_marker(marker_name)
+		if marker == null:
+			continue
+		var target: Dictionary = STAND_POSE[marker_name]
+		marker.position = target.get("pos", marker.position)
+		if marker_name == "Torso":
+			marker.rotation = target.get("rot", marker.rotation)
+	# Re-show the kinematic puppet and re-enable IK.
+	if _skeleton != null and is_instance_valid(_skeleton) and _skeleton.modification_stack != null:
+		_skeleton.modification_stack.enabled = true
+	if _body_container != null and is_instance_valid(_body_container):
+		_body_container.visible = true
+		_body_container.modulate.a = 1.0
+	state = RigState.ANIMATED
+	state_changed.emit(int(state))
+
+
 ## Applies the same velocity delta to every ragdoll body via a mass-scaled
 ## central impulse, preserving the ragdoll's internal structure. No-op outside
 ## RAGDOLL mode. Used by the physics harness "Knock Up" button.