Add save migration fixtures and failure-mode coverage

src/test/fixtures/saves/campaign_0_8.json

@@ -0,0 +1,11 @@
+{
+	"index": 1,
+	"next_venue": "joke_shop",
+	"shift_number": 2,
+	"difficulty": "hard",
+	"complete": false,
+	"completed": [
+		"hollywood_building"
+	],
+	"last_grade": "B"
+}