feat(mods): resume selected checkpoints from title

src/core/Checkpoint.lua

@@ -246,7 +246,7 @@ end
 
 local FACINGS = { up = true, down = true, left = true, right = true }
 
-local function validate(game, checkpoint)
+local function validate(game, checkpoint, expectedIdentity)
   if type(checkpoint) ~= "table" then
     return nil, "invalid_checkpoint", "Checkpoint root must be a table."
   end
@@ -264,13 +264,16 @@ local function validate(game, checkpoint)
   end
   local identity = copy.identity
   local current = game and game.save
-  local currentId = current and current.meta and current.meta.playthroughId
+  local currentId = expectedIdentity and expectedIdentity.playthroughId
+    or (current and current.meta and current.meta.playthroughId)
+  local currentVersion = expectedIdentity and expectedIdentity.gameVersion
+    or (current and current.version)
   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
-  if identity.gameVersion ~= current.version then
+  if identity.gameVersion ~= currentVersion then
     return nil, "wrong_game", "Checkpoint belongs to another game version."
   end
   if identity.playthroughId ~= currentId then
@@ -421,4 +424,81 @@ function Checkpoint.restore(game, checkpoint)
   return false, "restore_failed", "Checkpoint restoration failed: " .. tostring(err)
 end
 
+local function isTitleSession(game)
+  local states = game and game.stack and game.stack.states
+  if type(states) ~= "table" then return false end
+  for _, state in ipairs(states) do
+    if type(state) == "table" and state.screenId == "TitleState" then return true end
+  end
+  return false
+end
+
+local function emitRestored(game, checkpoint)
+  if ModRuntime.wants("checkpoint.restored") then
+    ModRuntime.emit("checkpoint.restored", { game = game, kind = checkpoint.kind })
+  end
+end
+
+local function rebuildTitle(game, savedTitle, rng)
+  local save, err = dataCopy(savedTitle)
+  if not save then error("title rollback decode failed: " .. tostring(err), 0) end
+  game.save = save
+  if type(game.adoptSave) == "function" then game:adoptSave(save) end
+  restoreRng(rng)
+  if not (game.stack and game.stack.top and game.stack.pop and game.stack.push
+      and type(game.makeTitleState) == "function") then
+    error("title recovery is unavailable", 0)
+  end
+  while game.stack:top() do game.stack:pop() end
+  game.stack:push(game:makeTitleState())
+end
+
+-- Reconstruct a validated persistent checkpoint from the title session. This
+-- is intentionally separate from restore(): title has no live gameplay state
+-- to capture for rollback. Validation happens before any mutation; a failed
+-- reconstruction rebuilds a fresh usable title session instead of exposing a
+-- half-installed overworld or battle.
+function Checkpoint.resume(game, checkpoint)
+  if not isTitleSession(game) then
+    return false, "not_at_title",
+      "Checkpoint resume is available only from the title session."
+  end
+  local save = game and game.save
+  local playthroughId, identityCode, identityMessage =
+    SaveData.selectedPlaythroughId(save)
+  if type(playthroughId) ~= "string" or playthroughId == "" then
+    return false, identityCode, identityMessage
+  end
+  local expected = { gameVersion = save and save.version, playthroughId = playthroughId }
+  local validated, code, message = validate(game, checkpoint, expected)
+  if not validated then return false, code, message end
+
+  local titleSave, titleErr = dataCopy(save)
+  if not titleSave then
+    return false, "title_recovery_unavailable",
+      "Could not preserve the title session: " .. tostring(titleErr)
+  end
+  local titleRng = captureRng()
+  local currentOptions = save.options
+  local ok, err = pcall(apply, game, validated, currentOptions)
+  if ok then
+    local restored, verifyCode = Checkpoint.capture(game)
+    if restored and validated.rng == nil then restored.rng = nil end
+    if restored and equalData(restored, validated) then
+      emitRestored(game, validated)
+      return true
+    end
+    err = restored and ("resumed state differed at "
+      .. tostring(firstDifference(validated, restored) or "canonical encoding"))
+      or ("resumed state could not be captured: " .. tostring(verifyCode))
+  end
+
+  local recovered, recoveryErr = pcall(rebuildTitle, game, titleSave, titleRng)
+  if not recovered then
+    return false, "title_recovery_failed",
+      "Checkpoint resume failed and title recovery failed: " .. tostring(recoveryErr)
+  end
+  return false, "resume_failed", "Checkpoint resume failed: " .. tostring(err)
+end
+
 return Checkpoint

src/core/SaveData.lua

@@ -967,6 +967,33 @@ function SaveData.ensurePlaythroughId(save, injectedFs)
   return id
 end
 
+-- Resolve the already-selected playthrough without changing the supplied save
+-- or allocating a replacement id. Title tools use this before a normal SAVE:
+-- Game:load intentionally owns a fresh skeleton there, while the selected
+-- launcher slot's durable tool data remains bound by the engine-owned mapping.
+-- This does not make arbitrary identities addressable; callers still need a
+-- higher-level engine capability that decides when selected resolution is safe.
+function SaveData.selectedPlaythroughId(save, injectedFs)
+  if type(save) ~= "table" then
+    return nil, "not_in_playthrough", "No selected playthrough is available."
+  end
+  local version = save.version or GameVersion.get()
+  if not knownVersion(version) then
+    return nil, "unknown_game", "The selected game version is unavailable."
+  end
+  local id = save.meta and save.meta.playthroughId
+  if type(id) == "string" and id ~= "" then return id end
+
+  local opts = SaveData.loadOptions(injectedFs)
+  local byVersion = opts.playthroughIds and opts.playthroughIds[version]
+  id = byVersion and byVersion[playthroughScope(version, injectedFs)] or nil
+  if type(id) ~= "string" or id == "" then
+    return nil, "no_selected_playthrough",
+      "The selected playthrough has no durable tool state."
+  end
+  return id
+end
+
 -- ------- meta
 
 -- the version/engine/mod-set stamp every v2 save carries; mods is the