Fix five multiplayer sync bugs, add automated two-instance test

Adds test/multiPlayerTest.tscn plus a driver that runs the kitchen flow
across two game instances: grab/drop, dirt station, sink washing, hob
cooking, counter combining and plating. Runs headless (run_mp_test.ps1),
in two visible windows (run_mp_test_windowed.ps1), or by hand with
keyboard controls (play_mp_test.ps1). 108 checks, exits non-zero on
failure.

After every step both peers snapshot every item's position and rendered
state and the server diffs them. Targeted assertions only look at the
thing a step touched, which misses desyncs elsewhere - that audit is
what caught the last bug below.

Bugs found and fixed:

- net_pickable: apply_held_state() only wrote `enabled` in its
  non-authority branch, so once a client grabbed an item every other
  peer set enabled=false and regaining authority never restored it. The
  server could then never pick that item up again, and a station would
  "snap" it (emitting has_picked_up, so a plate still got marked dirty)
  while pick_up() bailed out on the disabled item - leaving the zone
  holding an item with no grab driver.

- network_manager: station gating only happened in the spawn path, so
  stations baked into a scene file kept running their snap zones on
  clients and grabbed items straight out of the local hand. Added
  gate_existing_stations().

- network_manager: despawn_item() only freed the server's copy. Items
  baked into a scene aren't tracked by the MultiplayerSpawner, so
  consuming one left a ghost on every client, which then blocked the
  station it sat in and got grabbed instead of its replacement.

- network_manager: the snap-into-station decision read the server's own
  copy of the item position, but the reliable release RPC routinely
  overtakes the synchronizer's unordered position updates - so it acted
  on a stale position and teleported items back into the station they
  had just been carried away from. The releasing peer now sends its
  final transform and the server adopts it first.

- container: contained_ids.append()/erase() mutate the array in place,
  which never fires the setter that rebuilds the plate's visuals. The
  peer that put food on a plate was the only peer that never redrew it;
  remote peers looked right because the synchronizer assigns there.

Also null-guards XRServer.get_tracker() in the vendored xr-tools hand
grab point, which threw on every successful grab without an XR runtime,
and adds multiplayer_world.populate_from_layout so debug scenes can bake
their own content instead of spawning the whole kitchen.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Net/net_pickable.gd

@@ -20,6 +20,10 @@ var _pickable: XRToolsPickable
 # apply_held_state() last forced it to while non-authority.
 var _original_freeze_mode: int
 
+# Same idea for the pickable's authored `enabled` flag, which the non-authority
+# branch of apply_held_state() clears while someone else is holding the item.
+var _original_enabled: bool
+
 
 func _ready() -> void:
 	_pickable = get_parent() as XRToolsPickable
@@ -27,6 +31,7 @@ func _ready() -> void:
 		push_error("NetPickable must be a child of an XRToolsPickable")
 		return
 	_original_freeze_mode = _pickable.freeze_mode
+	_original_enabled = _pickable.enabled
 	_pickable.picked_up.connect(_on_picked_up)
 	_pickable.dropped.connect(_on_dropped)
 	# Deferred: the pickable root captures its own original_collision_mask/
@@ -72,6 +77,22 @@ func apply_held_state() -> void:
 				)
 			_pickable.freeze_mode = _original_freeze_mode
 			_pickable.collision_mask = _pickable.original_collision_mask
+		# Unlike freeze/collision (which XRToolsPickable manages itself while
+		# held), `enabled` is only ever written by the non-authority branch
+		# below, so it must be restored here or it stays false forever: once a
+		# client grabbed this item, every other peer set enabled=false, and
+		# regaining authority left it that way. On the server that silently
+		# broke everything downstream — hands couldn't pick the item up again,
+		# and a station snap zone would "snap" it (emitting has_picked_up, so
+		# e.g. a plate still got marked dirty) while pick_up() bailed out on
+		# the disabled item, leaving the zone holding an item with no grab
+		# driver that then fell out of the station.
+		if _pickable.enabled != _original_enabled:
+			if NetworkManager.is_online():
+				print("%s: reclaiming ownership, restoring enabled %s->%s" % [
+					_pickable.name, _pickable.enabled, _original_enabled
+				])
+			_pickable.enabled = _original_enabled
 		return
 	# A net_held_by/position sync update can race ahead of the
 	# authority-handoff RPC that's about to confirm a grab we just made
@@ -132,8 +153,11 @@ func _on_dropped(_p) -> void:
 		print("%s dropped, reporting release to server (lin=%s ang=%s)" % [
 			_pickable.name, _pickable.linear_velocity, _pickable.angular_velocity
 		])
+	# Send our own final transform too: we were the authority until now, and the
+	# server's copy may not have received our last position sync yet.
 	NetworkManager.release_item_authority_from(
-		_pickable.get_path(), _pickable.linear_velocity, _pickable.angular_velocity
+		_pickable.get_path(), _pickable.linear_velocity, _pickable.angular_velocity,
+		_pickable.global_transform
 	)
 
 

Net/network_manager.gd

@@ -121,8 +121,31 @@ func spawn_item(scene_path: String, xform: Transform3D, node_name: String = "",
 ## every peer when a tracked node exits the tree on the authority, so this is
 ## the single seam for destroying spawned items (works offline too).
 func despawn_item(node: Node) -> void:
-	if owns_world() and is_instance_valid(node):
-		log_line("despawn_item: %s" % node.name)
+	if not owns_world() or not is_instance_valid(node):
+		return
+	log_line("despawn_item: %s" % node.name)
+	# Items that came from the ItemsSpawner are despawned on every peer
+	# automatically when they leave the tree here. Items baked into a scene file
+	# are unknown to the spawner, so their removal has to be broadcast
+	# explicitly — otherwise every client keeps a ghost copy of an item the
+	# server has consumed, which then blocks the station it was sitting in and
+	# gets grabbed instead of the real item that replaced it.
+	if is_online() and not _is_spawner_tracked(node):
+		_despawn_static_item.rpc(node.get_path())
+	node.queue_free()
+
+
+# Items the ItemsSpawner replicates live under its spawn path; anything else was
+# baked into the scene file and the spawner knows nothing about it.
+func _is_spawner_tracked(node: Node) -> bool:
+	return _content_root != null and _content_root.is_ancestor_of(node)
+
+
+@rpc("authority", "call_remote", "reliable")
+func _despawn_static_item(path: NodePath) -> void:
+	var node := get_node_or_null(path)
+	if node:
+		log_line("despawn_static_item: freeing %s (the server consumed it)" % node.name)
 		node.queue_free()
 
 
@@ -163,6 +186,19 @@ func _gate_station(node: Node) -> void:
 	log_line("gated station (non-owner peer): %s" % node.name)
 
 
+## Gate every station already sitting in the scene tree, for peers that don't
+## own world logic. Stations that arrive through spawn_item() are gated as they
+## are built (see _spawn_item_from_data), but ones baked into a scene file never
+## pass through there — leaving a client running its own snap zones, which then
+## grab items straight out of the local hand and fight the server's
+## authoritative placement. Idempotent, so it's safe on every session start.
+func gate_existing_stations() -> void:
+	if owns_world():
+		return
+	for station in get_tree().get_nodes_in_group("station"):
+		_gate_station(station)
+
+
 # --- Item grab-authority transfer -----------------------------------------
 
 ## Called by NetPickable when this peer grabs an item by hand. Godot rejects
@@ -209,27 +245,35 @@ func _grant_or_reject_item_authority(item_path: NodePath, sender: int) -> void:
 ## Called by NetPickable when this peer releases an item, forwarding its throw
 ## velocity so the server can resume simulating it. Same self-RPC issue as
 ## above: runs directly if we're the server.
-func release_item_authority_from(item_path: NodePath, lin: Vector3, ang: Vector3) -> void:
+func release_item_authority_from(item_path: NodePath, lin: Vector3, ang: Vector3, xform: Transform3D) -> void:
 	if is_server():
-		_do_release_item_authority(item_path, lin, ang, multiplayer.get_unique_id())
+		_do_release_item_authority(item_path, lin, ang, xform, multiplayer.get_unique_id())
 	else:
-		_release_item_authority_rpc.rpc_id(1, item_path, lin, ang)
+		_release_item_authority_rpc.rpc_id(1, item_path, lin, ang, xform)
 
 
 @rpc("any_peer", "reliable")
-func _release_item_authority_rpc(item_path: NodePath, lin: Vector3, ang: Vector3) -> void:
+func _release_item_authority_rpc(item_path: NodePath, lin: Vector3, ang: Vector3, xform: Transform3D) -> void:
 	if not is_server():
 		return
-	_do_release_item_authority(item_path, lin, ang, multiplayer.get_remote_sender_id())
+	_do_release_item_authority(item_path, lin, ang, xform, multiplayer.get_remote_sender_id())
 
 
 ## Runs on the server. If released next to a station, the server snaps it in
 ## (server-authoritative placement).
-func _do_release_item_authority(item_path: NodePath, lin: Vector3, ang: Vector3, sender: int) -> void:
+func _do_release_item_authority(item_path: NodePath, lin: Vector3, ang: Vector3, xform: Transform3D, sender: int) -> void:
 	log_line("release_item_authority: %s released by peer %d" % [str(item_path), sender])
 	_set_item_authority.rpc(item_path, 1)
 	var item := get_node_or_null(item_path)
 	if item is RigidBody3D:
+		# Adopt the releasing peer's own final transform rather than trusting our
+		# copy's. That peer was the item's authority right up to this moment, and
+		# its position updates travel on the synchronizer's separate, unordered
+		# channel — this reliable RPC routinely overtakes them, leaving our copy
+		# still sitting where the item was BEFORE the peer carried it away. The
+		# snap decision below then reads that stale position and teleports the
+		# item straight back into the station it was just picked up from.
+		item.global_transform = xform
 		item.freeze = false
 		item.linear_velocity = lin
 		item.angular_velocity = ang