feat: expose storage engine compatibility context

docs/modding.md

@@ -158,6 +158,10 @@ local keys, code, message = mod.storage:list(game, "history/quick")
 local deleted, code, message = mod.storage:delete(game, "history/quick/q0001")
 ```
 
+`context` returns `{ engineVersion, gameVersion, playthroughId }`. The engine
+version is compatibility metadata; physical launcher-slot and path identity stays
+private.
+
 Values must be tables containing serializable data only. Keys are conservative
 slash-separated segments (letters, digits, `_`, `-`); paths and filesystem
 handles are never exposed. Writes are staged and decode-verified, reads recover

docs/rfcs/0003-playthrough-storage.md

@@ -51,10 +51,11 @@ honors injected test filesystems; it is not exposed on the mod object.
 Returns:
 
 ```lua
-{ gameVersion = "red", playthroughId = "..." }
+{ engineVersion = "0.9.0", gameVersion = "red", playthroughId = "..." }
 ```
 
-or `nil, code, message`. It intentionally omits launcher slot ids and paths.
+or `nil, code, message`. `engineVersion` is warning-grade compatibility metadata;
+the context intentionally omits launcher slot ids and paths.
 
 ### `mod.storage:write(game, key, value)`
 

src/mods/Storage.lua

@@ -3,6 +3,7 @@
 
 local SaveData = require("src.core.SaveData")
 local SaveSerializer = require("src.core.SaveSerializer")
+local Version = require("src.core.Version")
 
 local Storage = {}
 Storage.__index = Storage
@@ -81,7 +82,11 @@ end
 function Storage:context(game)
   local scope, code, message = self:_scope(game)
   if not scope then return nil, code, message end
-  return { gameVersion = scope.gameVersion, playthroughId = scope.playthroughId }
+  return {
+    engineVersion = Version.engine,
+    gameVersion = scope.gameVersion,
+    playthroughId = scope.playthroughId,
+  }
 end
 
 function Storage:_names(game, key, allowEmpty)

tests/modkit/cases/storage.lua

@@ -7,6 +7,7 @@ love = love or require("tests.love_stub")
 local T = require("tests.harness").suite("mod storage")
 local Loader = require("src.mods.Loader")
 local Runtime = require("src.mods.Runtime")
+local Version = require("src.core.Version")
 
 local savedEvents, savedHooks = Runtime.events, Runtime.hooks
 
@@ -90,8 +91,11 @@ end
 
 -- Removing scope identity or exposing a mutable private slot id breaks this.
 local context = alpha:context(current)
-T.same(context, { gameVersion = "red", playthroughId = "play-a" },
-  "context exposes stable game/playthrough identity only")
+T.same(context, {
+  engineVersion = Version.engine,
+  gameVersion = "red",
+  playthroughId = "play-a",
+}, "context exposes stable engine/game/playthrough compatibility identity")
 
 -- Data-only write/read. The literal expected table is independent of storage.
 local payload = { format = 1, nested = { money = 1234 }, flags = { a = true } }