docs: refresh save-format section against current validation/migration behavior

docs/DESIGN.md

@@ -46,7 +46,7 @@ The world grid is dynamically generated at runtime — `build_world()` creates a
 
 `game_state.gd` exposes `save_game()`, `load_game()`, `save_settings()`, `load_settings()`, and `clear_game()`. Desktop builds write JSON to `user://windowstead.save` and `user://windowstead.settings`. Web builds use `localStorage` via `JavaScriptBridge`.
 
-Save format is a single JSON dictionary with keys: `tick`, `harvested`, `resources`, `priority_order`, `dock_anchor`, `workers`, `tiles`, `builds`, `next_build_id`, `events`, `active_goal`, `completed_goal_ids`, `save_version`.
+Save format is a single JSON dictionary. `validate_save_schema()` in `scripts/game_state.gd` is the source of truth for the validated fields. Persisted keys: `tick`, `harvested`, `resources`, `priority_order`, `colony_stance`, `dock_anchor`, `workers`, `tiles`, `builds`, `next_build_id`, `reserved_resources`, `events`, `active_goal`, `completed_goal_ids`, `active_rewards`, `current_milestone_id`, `completed_milestone_ids`, `save_version`, plus `migration_log` on saves upgraded from version 1.
 
 ### Grid dimensions
 
@@ -73,6 +73,7 @@ The window is borderless + always-on-top by default. Transparent window mode is
   "harvested": {"wood": int, "stone": int, "food": int},
   "resources": {"wood": int, "stone": int, "food": int},
   "priority_order": ["build", "haul", "gather"],
+  "colony_stance": String,
   "dock_anchor": "bottom" | "left" | "right",
   "workers": [
     {
@@ -82,7 +83,7 @@ The window is borderless + always-on-top by default. Transparent window mode is
       "carrying": {"<resource>": int},
       "task": {"kind": String, "target": {"x": int, "y": int}, "resource": String, "build_id": int},
       "break_ticks": int,
-      "spawn_tick": int
+      "spawn_tick": int   // optional; set on recruit, backfilled by the v1 → 2 migration
     }
   ],
   "tiles": [
@@ -103,6 +104,16 @@ The window is borderless + always-on-top by default. Transparent window mode is
     "reward": String      // Optional reward label
   },
   "completed_goal_ids": [String],
+  "active_rewards": [
+    {"type": String, "remaining": int, "duration": int,
+     "label": String, "trickle_ticks": int, "resource": String}  // "resource" only on trickle rewards
+  ],
+  "current_milestone_id": String,
+  "completed_milestone_ids": [String],
+  "reserved_resources": {"<resource>": int},
+  "migration_log": [
+    {"from_version": int, "to_version": int, "step": String}
+  ],
   "save_version": int
 }
 ```
@@ -129,8 +140,10 @@ Each structure has build costs in wood and stone. Workers must haul resources to
 
 ## Save format
 
-- Versioned with `save_version` (currently 2).
-- On load, version mismatch triggers a colony reset.
+- Versioned with `save_version` (currently 2). A save with no `save_version` key is treated as the current version.
+- On load, every save passes through `validate_save_schema()` and then `migrate_save()` (shared by the desktop and web paths).
+- Version 1 saves are migrated in place to version 2 by `migrate_v1_to_v2()` and load normally — nothing is reset.
+- Future or unknown versions (greater than the current `SAVE_VERSION`) and version 0 are rejected: `migrate_save()` returns an empty dictionary, the load fails with an error message, and the running colony is left untouched. No reset happens on any load path.
+- `migrate_v1_to_v2()` writes `migration_log` (appending a step entry) and backfills `spawn_tick` on workers that lack it. The other newer fields (`active_goal`, `completed_goal_ids`, `active_rewards`, `current_milestone_id`, `completed_milestone_ids`, `colony_stance`, `reserved_resources`) are backfilled separately by `ColonySim.ensure_defaults()` after load, not by the migration.
 - Layout compatibility is checked: tile array size must match current grid, worker/build positions must be in bounds.
 - Settings are stored separately from game state.
-- Save version 1 → 2 migration adds `active_goal`, `completed_goal_ids`, and `spawn_tick` fields.