feat: anchor first checkpoint for cold restart

src/core/Checkpoint.lua

@@ -388,6 +388,47 @@ end
 
 local emitRestored
 
+-- Persist the current verified checkpoint as the ordinary progress anchor only
+-- when this playthrough has never had one. This is intentionally idempotent:
+-- durable checkpoint tools can make a first session resumable without turning
+-- every later checkpoint into a hidden normal SAVE. The live runtime must still
+-- match the supplied checkpoint, and the ordinary save.write veto/lifecycle
+-- remains authoritative through Game:writeSave().
+function Checkpoint.ensureNormalSave(game, checkpoint, injectedFs)
+  local capability = Checkpoint.inspect(game)
+  if not capability.canCapture then
+    return false, capability.reason, capability.message
+  end
+  local validated, code, message = validate(game, checkpoint)
+  if not validated then return false, code, message end
+
+  local info, infoCode, infoMessage =
+    SaveData.selectedNormalSaveInfo(game.save, injectedFs)
+  if not info then return false, infoCode, infoMessage end
+  if info.exists then return true, "already_exists" end
+
+  local current, captureCode, captureMessage = Checkpoint.capture(game)
+  if not current then return false, captureCode, captureMessage end
+  if not equalData(current, validated) then
+    return false, "checkpoint_not_current",
+      "The active runtime changed after this checkpoint was captured."
+  end
+  if type(game.writeSave) ~= "function" then
+    return false, "save_unavailable",
+      "The active runtime cannot persist ordinary progress."
+  end
+  local ok, saved = pcall(game.writeSave, game)
+  if not ok or saved == false then
+    return false, "save_failed", "Could not create the first ordinary progress save."
+  end
+  local verified = SaveData.selectedNormalSaveInfo(game.save, injectedFs)
+  if type(verified) ~= "table" or not verified.exists then
+    return false, "save_verify_failed",
+      "The first ordinary progress save could not be verified."
+  end
+  return true
+end
+
 function Checkpoint.restore(game, checkpoint)
   local capability = Checkpoint.inspect(game)
   if not capability.canRestore then

src/core/SaveData.lua

@@ -941,7 +941,17 @@ local function rememberPlaythroughId(save, opts, injectedFs)
   if type(id) ~= "string" or id == "" then return opts, false end
   local version = save.version or GameVersion.get()
   local scope = playthroughScope(version, injectedFs)
-  opts = opts or SaveData.loadOptions(injectedFs)
+  local persisted = SaveData.loadOptions(injectedFs)
+  if opts then
+    -- Slot selection and opaque playthrough routing are engine-owned launcher
+    -- state. A live game may carry an options snapshot from before a legacy
+    -- save was promoted to slot1; writing that stale snapshot must not erase
+    -- the freshly persisted routing and strand tool storage on next boot.
+    opts.saveSlots = deepCopy(persisted.saveSlots)
+    opts.playthroughIds = deepCopy(persisted.playthroughIds)
+  else
+    opts = persisted
+  end
   opts.playthroughIds = opts.playthroughIds or {}
   opts.playthroughIds[version] = opts.playthroughIds[version] or {}
   local changed = opts.playthroughIds[version][scope] ~= id
@@ -1021,18 +1031,18 @@ function SaveData.selectedNormalSaveInfo(save, injectedFs)
     or readTable(fs, staged)
     or readTable(fs, backup)
   if type(normal) ~= "table" or normal.version ~= version then
-    return { savedAt = nil }
+    return { exists = false, savedAt = nil }
   end
   local normalId = normal.meta and normal.meta.playthroughId
   if type(normalId) == "string" and normalId ~= "" and normalId ~= playthroughId then
-    return { savedAt = nil }
+    return { exists = false, savedAt = nil }
   end
   local savedAt = normal.meta and normal.meta.savedAt
   if type(savedAt) ~= "number" or savedAt < 0 or savedAt ~= savedAt
       or savedAt == math.huge or savedAt == -math.huge then
     savedAt = nil
   end
-  return { savedAt = savedAt }
+  return { exists = true, savedAt = savedAt }
 end
 
 -- ------- meta

src/mods/Loader.lua

@@ -683,6 +683,9 @@ function Loader:_api(mod)
       resume = function(_, game, checkpoint)
         return Checkpoint.resume(game, checkpoint)
       end,
+      ensureNormalSave = function(_, game, checkpoint)
+        return Checkpoint.ensureNormalSave(game, checkpoint, loader.fs)
+      end,
     },
     options = {
       define = function(_, schema)