Gen 2: a cart saves into its own scope, not the base game's

A cart's saves are keyed by cart id, not by version.  SaveData resolves
every path through activeScopeKey, which answers cart_<id> while one is
active, and the launcher lists, creates and selects a cart's slots from
the cartSlots registry (RomImporter._refreshSlots / _selectSlot /
_newSlot).

src/core/gen2/Save.lua asked in the version's name alone.  saveNames
built saves/<version>/<slot>.lua or save_<suffix>.lua from the version in
both branches and never consulted the active cart, so a cart on Gold,
Silver or Crystal read and wrote the BASE GAME's playthrough.  Gen 1 was
unaffected because it saves through SaveData itself, which is already
cart-scoped -- so this only showed on a Gen 2 cart.

It was worse than sharing one file.  Save.save opens by asking
activeSlot(version) and, on nil, calling createSlot + setActiveSlot in
the version's name, so the first save inside a cart registered a slot in
the base game's registry and made it active: the cart's playthrough
appeared in the launcher's list for the base version, and the player's
own save there was what the cart then overwrote.

Both sites now resolve the cart scope the way SaveData does, and the
names they build are SaveData's own -- slotDir's saves/cart_<id>/ and
legacyNames' save_cart_<id>.lua -- so the in-game save layer and the
launcher land on one file again.

tests/gen2_save_test.lua covers the cart's flat name, its slot name, the
slot going into the cart's registry rather than the base game's, the
cart's scope winning over a base slot, and the base game keeping its own
once the cart is cleared.  Four of them fail on the unpatched module.

Reported as "when I select Wild Crystal to launch it loads my save from
regular Crystal".

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

src/core/gen2/Save.lua

@@ -118,13 +118,34 @@ local function saveNames(version)
   -- no save and dropped CONTINUE (and a following SAVE wrote a second copy to
   -- the flat path the launcher no longer looks at).  Reading through the same
   -- slot resolution keeps the in-game load/save and the launcher on one file.
+  --
+  -- Resolve the SCOPE the same way as well.  A cart's saves are keyed by cart
+  -- id rather than by version: SaveData resolves every path through
+  -- `activeScopeKey`, which answers `cart_<id>` while one is active, and the
+  -- launcher lists, creates and selects a cart's slots from the `cartSlots`
+  -- registry (RomImporter._refreshSlots / _selectSlot / _newSlot).  This
+  -- module asked in the version's name alone, so a Gen 2 cart read and wrote
+  -- the BASE GAME's playthrough.  Gen 1 was unaffected because it saves
+  -- through SaveData, which is already cart-scoped.
   local ok, SaveData = pcall(require, "src.core.SaveData")
-  local slot = ok and SaveData.activeSlot and SaveData.activeSlot(version) or nil
+  SaveData = ok and SaveData or nil
+  local cart = SaveData and SaveData.getCart and SaveData.getCart() or nil
+  if type(cart) ~= "string" or cart == "" then cart = nil end
+  local slot
+  if cart then
+    slot = SaveData.activeCartSlot and SaveData.activeCartSlot(cart) or nil
+  elseif SaveData then
+    slot = SaveData.activeSlot and SaveData.activeSlot(version) or nil
+  end
   if slot then
-    local main = "saves/" .. version .. "/" .. slot .. ".lua"
+    local scope = cart and ("cart_" .. cart) or version
+    local main = "saves/" .. scope .. "/" .. slot .. ".lua"
     return main, main .. ".bak", main .. ".tmp"
   end
-  local main = "save" .. GameVersion.saveSuffix(version) .. ".lua"
+  -- SaveData.legacyNames names a cart "save_cart_<id>.lua" and a version by
+  -- its own suffix; this is that, so the two ends agree before slots exist.
+  local suffix = cart and ("_cart_" .. cart) or GameVersion.saveSuffix(version)
+  local main = "save" .. suffix .. ".lua"
   return main, main .. ".bak", main .. ".tmp"
 end
 
@@ -911,7 +932,21 @@ function Save.save(save)
   local version = save.version
   do
     local ok, SaveData = pcall(require, "src.core.SaveData")
-    if ok and SaveData.activeSlot and not SaveData.activeSlot(version) then
+    -- In the CART's registry when one is active, for the same reason
+    -- saveNames resolves the cart's scope: otherwise the first save inside a
+    -- cart registers a slot against the base game and makes it active, so the
+    -- cart's playthrough appears in the launcher's list for the base version
+    -- and the player's own save there is what it overwrites.
+    local cart = ok and SaveData.getCart and SaveData.getCart() or nil
+    if type(cart) ~= "string" or cart == "" then cart = nil end
+    if ok and cart then
+      if SaveData.activeCartSlot and not SaveData.activeCartSlot(cart) then
+        local id = SaveData.createCartSlot and SaveData.createCartSlot(cart)
+        if id and SaveData.setActiveCartSlot then
+          SaveData.setActiveCartSlot(cart, id)
+        end
+      end
+    elseif ok and SaveData.activeSlot and not SaveData.activeSlot(version) then
       local id = SaveData.createSlot and SaveData.createSlot(version)
       if id and SaveData.setActiveSlot then
         SaveData.setActiveSlot(version, id)