feat: synchronize authoritative npc position

- NpcVisual emits position_changed signal during _physics_process
- SimulationManager.synchronize_npc_position() writes active position
  into SimNPC state on movement, arrival, and navigation failure
- WorldViewManager forwards position_changed and syncs before
  despawn/reload
- SimNPC stores travel_target_position and has_travel_target as
  versioned state; NPCStateRecord v2 persists them with v1 migration
- WorldViewManager.spawn_npc_visual() resumes travel for traveling NPCs
  via SimulationManager.request_current_travel()
- WorldViewManager.despawn_npc_visual() syncs position before removal
- WorldViewManager.reload_npc_visual() spawns a visual from persisted
  state without rerolling target selection
- tests/npc_visual_lifecycle_test.gd proves unload/reload preserves
  action, target, reservation, position, RNG state, and checksum
- simulation_state_serialization_test adds legacy v1 NPC migration test
  that does not invent an active travel target

tests/npc_visual_lifecycle_test.gd

@@ -0,0 +1,94 @@
+extends SceneTree
+
+var failures: Array[String] = []
+
+func _initialize() -> void:
+	call_deferred("_run")
+
+func _run() -> void:
+	var main_scene: Node = load("res://main.tscn").instantiate()
+	root.add_child(main_scene)
+	await process_frame
+	await physics_frame
+	await physics_frame
+
+	var manager: Node = main_scene.get_node("SimulationManager")
+	var view: Node = main_scene.get_node("WorldViewManager")
+	manager.set_process(false)
+	var npc: SimNPC = manager.npcs[0]
+	var visual: Node3D = view.active_npc_visuals[npc.id]
+	visual.set_physics_process(false)
+
+	var resource_state: ResourceStateRecord = manager.get_resource_state(
+		&"berry_bush_01"
+	)
+	npc.set_task(SimulationIds.ACTION_GATHER_FOOD)
+	npc.target_id = resource_state.get_node_id()
+	npc.travel_target_position = Vector3(-6.0, 0.0, -8.0)
+	npc.has_travel_target = true
+	resource_state.reserve(npc.id)
+	manager.request_current_travel(npc.id)
+
+	var saved_action := npc.current_task
+	var saved_target_id := npc.target_id
+	var saved_target_position := npc.travel_target_position
+	var authoritative_position := Vector3(-2.5, 0.5, 1.75)
+	visual.global_position = authoritative_position
+
+	_check(view.despawn_npc_visual(npc.id), "Visual should unload")
+	await process_frame
+	_check(
+		npc.position.is_equal_approx(authoritative_position),
+		"Unload should synchronize the last active position"
+	)
+	_check(
+		npc.current_task == saved_action
+		and npc.target_id == saved_target_id
+		and npc.travel_target_position.is_equal_approx(saved_target_position),
+		"Unload should preserve action and target state"
+	)
+	_check(
+		resource_state.get_reserved_by() == npc.id,
+		"Unload should preserve the resource reservation"
+	)
+	var unloaded_checksum: String = manager.get_state_checksum()
+
+	_check(view.reload_npc_visual(npc.id), "Visual should reload")
+	var reloaded: Node3D = view.active_npc_visuals[npc.id]
+	reloaded.set_physics_process(false)
+	_check(
+		reloaded.global_position.is_equal_approx(authoritative_position),
+		"Reloaded visual should use authoritative position"
+	)
+	var resumed_target: Vector3 = reloaded.get("current_target")
+	_check(
+		resumed_target.is_equal_approx(saved_target_position),
+		"Reloaded visual should resume the persisted travel target"
+	)
+	_check(
+		manager.get_state_checksum() == unloaded_checksum,
+		"Loading a visual must not mutate simulation state"
+	)
+	_check(
+		resource_state.get_reserved_by() == npc.id,
+		"Reload should preserve the reservation"
+	)
+
+	var moved_position := authoritative_position + Vector3(0.5, 0.0, 0.25)
+	reloaded.emit_signal("position_changed", npc.id, moved_position)
+	_check(
+		npc.position.is_equal_approx(moved_position),
+		"Active position callback should synchronize simulation state"
+	)
+
+	if failures.is_empty():
+		print("[TEST] NPC visual lifecycle passed")
+		quit(0)
+		return
+	for failure in failures:
+		push_error("[TEST] " + failure)
+	quit(1)
+
+func _check(condition: bool, message: String) -> void:
+	if not condition:
+		failures.append(message)