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/savestreampsp.h

@@ -53,3 +53,12 @@ errorret_t saveStreamWriteBytesPSP(
  * @return An error if pos is out of range.
  */
 errorret_t saveStreamSeekPSP(savestreampsp_t *p, const size_t pos);
+
+/**
+ * Gets the current read/write position within the buffer.
+ *
+ * @param p   Active stream.
+ * @param out Receives the current position.
+ * @return An error - always succeeds, matches saveStreamTellImpl's shape.
+ */
+errorret_t saveStreamTellPSP(savestreampsp_t *p, size_t *out);