feat: add validated quicksave persistence

tests/npc_visual_lifecycle_test.gd

@@ -99,6 +99,23 @@ func _run() -> void:
 		"Active position callback should synchronize simulation state"
 	)
 
+	var saved_state: String = manager.serialize_state()
+	var visual_before_restore: Node3D = view.active_npc_visuals[npc.id]
+	npc.hunger = 99.0
+	_check(
+		manager.restore_state_from_json(saved_state),
+		"Visual lifecycle scenario should restore valid state"
+	)
+	var visual_after_restore: Node3D = view.active_npc_visuals[npc.id]
+	_check(
+		visual_after_restore != visual_before_restore,
+		"State restoration should rebuild active NPC presentation"
+	)
+	_check(
+		visual_after_restore.global_position.is_equal_approx(npc.position),
+		"Rebuilt presentation should use the restored authoritative position"
+	)
+
 	if failures.is_empty():
 		print("[TEST] NPC visual lifecycle passed")
 		quit(0)

tests/save_slot_persistence_test.gd

@@ -0,0 +1,75 @@
+extends SceneTree
+
+var failures: Array[String] = []
+
+func _initialize() -> void:
+	call_deferred("_run")
+
+func _run() -> void:
+	var test_directory := "user://save_slot_test_%s" % Time.get_ticks_usec()
+	var store := SaveSlotStore.new(test_directory)
+	var manager: Node = load("res://simulation/SimulationManager.gd").new()
+	manager.debug_logs = false
+	root.add_child(manager)
+	manager.set_process(false)
+
+	for step in 8:
+		manager.simulate_tick()
+	var saved_checksum: String = manager.get_state_checksum()
+	_check(store.save(manager), "A valid simulation should save atomically")
+	_check(store.has_slot(), "The validated quicksave should be discoverable")
+
+	for step in 5:
+		manager.simulate_tick()
+	_check(
+		manager.get_state_checksum() != saved_checksum,
+		"The scenario should change after the save point"
+	)
+	_check(store.load_into(manager), "A valid quicksave should load")
+	_check(
+		manager.get_state_checksum() == saved_checksum,
+		"Loading should restore the exact saved simulation"
+	)
+
+	var final_path := store.get_slot_path()
+	var backup_path := final_path + ".bak"
+	_check(
+		DirAccess.rename_absolute(final_path, backup_path) == OK,
+		"Test should reproduce the atomic replacement recovery window"
+	)
+	_check(
+		store.load_into(manager),
+		"A preserved previous save should recover an interrupted replacement"
+	)
+	_check(
+		DirAccess.rename_absolute(backup_path, final_path) == OK,
+		"Test should restore the primary save path"
+	)
+
+	var unchanged_checksum: String = manager.get_state_checksum()
+	var file := FileAccess.open(final_path, FileAccess.WRITE)
+	file.store_string("{\"schema\":\"broken\"}")
+	file.close()
+	_check(not store.load_into(manager), "Invalid save data should be rejected")
+	_check(
+		manager.get_state_checksum() == unchanged_checksum,
+		"A rejected save must not partially mutate the simulation"
+	)
+	_check(
+		not store.save(manager, "../escape"),
+		"Slot names must not escape the save directory"
+	)
+
+	store.delete_slot()
+	DirAccess.remove_absolute(ProjectSettings.globalize_path(test_directory))
+	if failures.is_empty():
+		print("[TEST] Save-slot persistence 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)

tests/save_slot_persistence_test.gd.uid

@@ -0,0 +1 @@
+uid://c7ow4dj42sfhl