Add save migration fixtures and failure-mode coverage

docs/getting-started.md

@@ -74,6 +74,32 @@ XDG_DATA_HOME=/tmp/gigline-campaign godot --path . --fixed-fps 60 --script res:/
 
 The unit test runner is the project's own lightweight harness (no editor plugin required) so CI can run it without the editor.
 
+## Save Data and Migration
+
+The game keeps four small JSON files in Godot's `user://` directory, which is
+`~/.local/share/godot/app_userdata/Gig Line/` on Linux,
+`%APPDATA%\Godot\app_userdata\Gig Line\` on Windows and
+`~/Library/Application Support/Godot/app_userdata/Gig Line/` on macOS:
+
+| File | Contents |
+| --- | --- |
+| `settings.json` | Camera, HUD, binding, difficulty, audio, accessibility and tutorial flags. Missing keys are filled from the current defaults; there is no schema version because the field set grew compatibly. |
+| `campaign.json` | Campaign index, completed venues, difficulty, shift number and last grade. Older files with missing fields open at the safe defaults. |
+| `shift_results.json` | Completed-shift review history, appended as an array. |
+| `shift_state.json` | Resumable mid-shift checkpoint, version `1`. It stores the venue, guard/door/flashlight/body-camera state, coordinator snapshot and NPC rows. |
+
+Writes go to a sibling `.tmp` file and are renamed only after the complete JSON
+is written. A corrupt or partial file is ignored on load, so the player gets
+defaults rather than a crash and a valid old file is not overwritten by a failed
+replace. The migration fixtures used by the tests live in
+`src/test/fixtures/saves/`.
+
+For a clean first launch, delete the four files (or the whole `Gig Line` user
+directory). Settings' **Reset campaign** clears campaign progress separately.
+To back up or move a profile, copy those files while the game is closed; to roll
+back, restore the previous copies. Shift history is diagnostic and is not
+required to resume a checkpoint.
+
 ## Controls
 
 Keyboard and mouse are the default; a gamepad works without configuration (left stick moves,