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/simulation_state_serialization_test.gd

@@ -8,6 +8,7 @@ func _initialize() -> void:
 func _run() -> void:
 	await _test_deterministic_continuation()
 	await _test_resource_state_round_trip()
+	_test_legacy_npc_migration()
 	_test_legacy_resource_migration()
 	_test_schema_rejection()
 
@@ -160,6 +161,27 @@ func _test_legacy_resource_migration() -> void:
 		"Resource migration should preserve mutable authority"
 	)
 
+func _test_legacy_npc_migration() -> void:
+	var npc := SimNPC.new(
+		88,
+		"LegacyNPC",
+		SimulationIds.PROFESSION_FARMER,
+		5.0,
+		5.0
+	)
+	var legacy_data := NPCStateRecord.capture(npc).to_dictionary()
+	legacy_data["schema_version"] = NPCStateRecord.LEGACY_SCHEMA_VERSION
+	legacy_data.erase("travel_target_position")
+	legacy_data.erase("has_travel_target")
+	var migrated := NPCStateRecord.from_dictionary(legacy_data)
+	_check(migrated != null, "NPCStateRecord v1 should migrate explicitly")
+	if migrated == null:
+		return
+	_check(
+		not bool(migrated.data["has_travel_target"]),
+		"Legacy NPC migration should not invent an active travel target"
+	)
+
 func _create_manager(seed_value: int) -> Node:
 	var manager: Node = load("res://simulation/SimulationManager.gd").new()
 	manager.simulation_seed = seed_value