Add save migration fixtures and failure-mode coverage

src/test/fixtures/saves/shift_state_0_9_0.json

@@ -0,0 +1,26 @@
+{
+	"version": 1,
+	"scenario_id": "joke_shop",
+	"guard_position": [0.0, 0.1, 10.5],
+	"guard_yaw": 1.25,
+	"head_pitch": 0.1,
+	"door_locked": true,
+	"flashlight_on": true,
+	"body_camera_on": true,
+	"npc_counter": 7,
+	"coordinator": {
+		"duration_s": 900.0,
+		"elapsed_s": 540.0,
+		"current_phase": 3,
+		"phase_offset_s": 700.0,
+		"spawned_count": 7,
+		"responder_outcomes": [],
+		"investigations": [],
+		"recorder": {},
+		"reputation": {},
+		"crowd": {},
+		"radio": {},
+		"scheduler": {}
+	},
+	"npcs": []
+}