Unify save system into one save.h/.c; diverge storage format per platform

Renames savefile_t to saveslot_t and folds last session's standalone
settings.h/.c module back in as savemeta_t, so there's one save system
(SAVE.slots[] + SAVE.meta) instead of two parallel ones - while letting
each platform pick its own physical format for the two concepts:

- Linux now writes human-editable JSON (slot0.json, settings.json, ...)
  via yyjson's mutable writer API, so players can hand-fix a bad setting.
- PSP folds meta into the same sceUtilitySavedata binary payload as its
  one save slot (SAVE_SLOT_COUNT_MAX=1 there - a future save picker will
  let players manage multiple named saves via the OS's own browser).
- GameCube consolidates the 3 per-slot memory card files and the separate
  settings file into one combined card file.

Also fixes two bugs surfaced while building this: the CRC finalize step
seeked to a hardcoded offset (only safe for one section per file, breaks
once meta+slots share a buffer), and save.c's async/sync dispatch left an
unconditional fallback call that doesn't exist on PSP-only platforms.

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

src/duskpsp/save/CMakeLists.txt

@@ -8,6 +8,12 @@ target_sources(${DUSK_LIBRARY_TARGET_NAME}
   PUBLIC
     savepsp.c
     savestreampsp.c
-    settingspsp.c
-    settingsstreampsp.c
+)
+
+# PSP only needs one Dusk-side save slot - a future main-menu save picker
+# will let players manage multiple named saves through the OS's own
+# sceUtilitySavedata browser instead of Dusk maintaining its own numbered
+# slots (see save/saveslot.h for the default used by every other platform).
+target_compile_definitions(${DUSK_LIBRARY_TARGET_NAME} PUBLIC
+  SAVE_SLOT_COUNT_MAX=1
 )