Add versioned save slot/settings schemas with strict version checks

Splits saveslot_t/savesettings_t into a stable, version-independent
in-memory struct (save/slot/, save/settings/) and a separate frozen
per-schema-version struct (saveslotver1_t/savesettingsver1_t) used only
for JSON serialization, converted between manually. Every write now
stamps a "version" field (SAVE_SLOT_CURRENT/SAVE_SETTINGS_CURRENT), and
every read requires it to match exactly, rejecting anything else -
laying the groundwork for a real migration path instead of silently
misreading old/incompatible save data.

src/dusklinux/save/savedevicelinux.c

@@ -7,8 +7,8 @@
 
 #include "save/savedevice.h"
 #include "save/savedevicelinux.h"
-#include "save/saveslot.h"
-#include "save/savesettings.h"
+#include "save/slot/saveslot.h"
+#include "save/settings/savesettings.h"
 #include "save/savejson.h"
 #include "asset/asset.h"
 #include "assert/assert.h"