Add save migration fixtures and failure-mode coverage

src/test/fixtures/saves/settings_0_8.json

@@ -0,0 +1,15 @@
+{
+	"look_sensitivity": 0.0031,
+	"hud_font_scale": 1.15,
+	"invert_y": true,
+	"head_bob": false,
+	"input_bindings": {
+		"inspect": {
+			"type": "key",
+			"code": 90
+		}
+	},
+	"ai_difficulty": "hard",
+	"tutorial_completed": true,
+	"tutorial_skipped": false
+}