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

player/npc/NpcVisual.gd

@@ -2,6 +2,7 @@ extends CharacterBody3D
 
 signal arrived_at_target(sim_id: int)
 signal navigation_failed(sim_id: int)
+signal position_changed(sim_id: int, active_position: Vector3)
 
 const DEBUG_LOGS := true
 
@@ -123,6 +124,8 @@ func _physics_process(delta: float) -> void:
 	velocity.y = 0.0
 
 	move_and_slide()
+	if not global_position.is_equal_approx(prev_pos):
+		position_changed.emit(sim_id, global_position)
 
 	var minimum_progress := move_speed * delta * 0.1
 	if global_position.distance_squared_to(prev_pos) < minimum_progress * minimum_progress: