Refuse a Gen 2 cart save as a Gen 2 save, not as a broken Gen 1 one

#1832 reports a Crystal .sav failing to import with "save data checksum
invalid (main data checksum mismatch)". The save is fine. It is being
measured with the wrong generation's ruler.

SaveFileIO.importToSlot sends anything that is not exactly SAVE_SIZE into
mainChecksumValid, which is pokered's main-data checksum. Gen 2 carts are
MBC3+TIMER, so a real Gold, Silver or Crystal battery save carries an RTC
footer: 32786 bytes, never 32768. It misses the size test, falls into the
checksum branch, and is told it is damaged by a rule written for a
different game. Every real Gen 2 cart save takes that path, every time.

The generation check already exists, it just lives too late, inside
importSav and behind the size gate that has already rejected the save.
This lifts it into SaveConvert.importSupported and exportSupported and
asks before the bytes are measured. Both existing guards route through
the same predicates now, so the early caller and the late one cannot
describe the same game two different ways.

Nothing changes for Gen 1, and Gen 2 is still refused, for the real
reason and in a sentence a player can do something with.

tests/gen2_save_import_message_test.lua covers it. Against the code as it
stands it is 20/26, and the failures are the report itself.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/import/SaveFileIO.lua

@@ -78,6 +78,12 @@ function SaveFileIO.importToSlot(source, version, force)
   version = version or GameVersion.get()
   local bytes, readErr = readSource(source)
   if not bytes then return false, readErr end
+  -- The GAME decides before the BYTES do.  Everything below this line judges a
+  -- save by Gen 1's rules -- the size test, and mainChecksumValid, which is
+  -- pokered's checksum -- so a Gen 2 cart save reaching it is measured against
+  -- a rule that cannot match and comes back "checksum invalid" (#1832).
+  local supported, unsupportedWhy = SaveConvert.importSupported(version)
+  if not supported then return false, unsupportedWhy end
   if #bytes ~= SAVE_SIZE then
     local check = SaveConvert.mainChecksumValid(bytes)
     if check == nil then

src/save_convert/SaveConvert.lua

@@ -235,6 +235,35 @@ local function gen2CartName(gameVersion)
   return GameVersion.info(gameVersion).displayName
 end
 
+-- Can a cart save for this game cross in or out at all?  Public because the
+-- launcher has to ask about the GAME before it measures the BYTES.
+--
+-- SaveFileIO.importToSlot judges anything that is not exactly SAVE_SIZE with
+-- mainChecksumValid, which is pokered's main-data checksum.  A Gen 2 cart is
+-- MBC3+TIMER, so a real Gold/Silver/Crystal .sav carries an RTC footer and is
+-- 32786 bytes: it misses the size test and is then measured against a rule
+-- that was never going to match it.  The player is told "save data checksum
+-- invalid" about a save that is perfectly good (#1832).
+--
+-- Returns true, or false plus the same sentence importSav/exportSav would have
+-- answered with, so a caller that asks early and a caller that does not cannot
+-- describe the same game two different ways.
+function SaveConvert.importSupported(gameVersion)
+  local gen2Name = gen2CartName(gameVersion)
+  if gen2Name then
+    return false, gen2Name .. " uses a Gen 2 cart save; importing one is not supported yet."
+  end
+  return true
+end
+
+function SaveConvert.exportSupported(gameVersion)
+  local gen2Name = gen2CartName(gameVersion)
+  if gen2Name then
+    return false, gen2Name .. " uses a Gen 2 cart save; exporting one is not supported yet."
+  end
+  return true
+end
+
 -- importSav(bytes, version, gameVersion) -> saveTable, err
 -- bytes: the raw 32768-byte SRAM string. Validates size and the main-data
 -- checksum, decodes through GenSave, and returns a save table fully merged
@@ -247,10 +276,8 @@ function SaveConvert.importSav(bytes, version, gameVersion)
   if type(bytes) ~= "string" then
     return nil, "expected raw save bytes as a string"
   end
-  local gen2Name = gen2CartName(gameVersion)
-  if gen2Name then
-    return nil, gen2Name .. " uses a Gen 2 cart save; importing one is not supported yet."
-  end
+  local supported, unsupportedWhy = SaveConvert.importSupported(gameVersion)
+  if not supported then return nil, unsupportedWhy end
   if #bytes ~= GenSave.SAVE_SIZE then
     return nil, ("save must be %d bytes, got %d"):format(GenSave.SAVE_SIZE, #bytes)
   end
@@ -283,10 +310,8 @@ function SaveConvert.exportSav(saveTable, gameVersion)
   if type(saveTable) ~= "table" then
     return nil, "expected a save table"
   end
-  local gen2Name = gen2CartName(gameVersion)
-  if gen2Name then
-    return nil, gen2Name .. " uses a Gen 2 cart save; exporting one is not supported yet."
-  end
+  local supported, unsupportedWhy = SaveConvert.exportSupported(gameVersion)
+  if not supported then return nil, unsupportedWhy end
   local data, derr = ensureData(gameVersion)
   if not data then return nil, derr end
   local ok, bytes = pcall(GenSave.encode, saveTable, data, nil)