feat: include engine version in checkpoints

docs/rfcs/0004-runtime-checkpoints.md

@@ -56,7 +56,9 @@ Returns a detached data-only format-1 checkpoint, or
 {
   format = 1,
   kind = "overworld",
-  identity = { gameVersion = "red", playthroughId = "..." },
+  identity = {
+    engineVersion = "...", gameVersion = "red", playthroughId = "...",
+  },
   save = { -- canonical dynamic progress, excluding global options },
   runtime = { overworld = {
     map = "PALLET_TOWN", x = 5, y = 6,
@@ -65,7 +67,8 @@ Returns a detached data-only format-1 checkpoint, or
 }
 ```
 
-Capture deep-copies through the restricted serializer before and after
+`engineVersion` is metadata for caller compatibility warnings; the engine does
+not reject patch/minor mismatches on restore. Capture deep-copies through the restricted serializer before and after
 `OverworldController:captureSave` synchronizes live map, tile, facing, and surf
 state. It excludes `save.options`, functions, userdata, threads, metatables as
 behavior, controller instances, and static content registries. Failure code

src/core/Checkpoint.lua

@@ -3,6 +3,7 @@
 
 local SaveSerializer = require("src.core.SaveSerializer")
 local SaveData = require("src.core.SaveData")
+local Version = require("src.core.Version")
 
 local Checkpoint = {}
 
@@ -119,6 +120,7 @@ function Checkpoint.capture(game)
     format = Checkpoint.FORMAT,
     kind = "overworld",
     identity = {
+      engineVersion = Version.engine,
       gameVersion = game.save.version,
       playthroughId = game.save.meta.playthroughId,
     },
@@ -154,7 +156,8 @@ local function validate(game, checkpoint)
   local identity = copy.identity
   local current = game and game.save
   local currentId = current and current.meta and current.meta.playthroughId
-  if type(identity) ~= "table" or type(identity.gameVersion) ~= "string"
+  if type(identity) ~= "table" or type(identity.engineVersion) ~= "string"
+      or type(identity.gameVersion) ~= "string"
       or type(identity.playthroughId) ~= "string" then
     return nil, "invalid_checkpoint", "Checkpoint identity is missing or corrupt."
   end

tests/modkit/cases/checkpoints.lua

@@ -9,6 +9,7 @@ local Loader = require("src.mods.Loader")
 local Runtime = require("src.mods.Runtime")
 local GameMethods = require("src.core.Game")
 local StateStack = require("src.core.StateStack")
+local Version = require("src.core.Version")
 
 local savedEvents, savedHooks = Runtime.events, Runtime.hooks
 
@@ -204,7 +205,11 @@ local snapshot, code, message = checkpoints:capture(game)
 T.check(snapshot ~= nil, "stable overworld captures: " .. tostring(code or message))
 T.eq(snapshot.format, 1, "checkpoint format is explicit")
 T.eq(snapshot.kind, "overworld", "checkpoint runtime kind is explicit")
-T.same(snapshot.identity, { gameVersion = "red", playthroughId = "play-a" },
+T.same(snapshot.identity, {
+    engineVersion = Version.engine,
+    gameVersion = "red",
+    playthroughId = "play-a",
+  },
   "checkpoint carries compatibility identity")
 T.same(snapshot.runtime.overworld,
   { map = "ROUTE_1", x = 7, y = 8, facing = "left", surfing = true },