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/duskdolphin/save/savestreamdolphin.h

@@ -20,34 +20,28 @@ typedef struct {
   size_t position;
   /** True when opened for writing; flushes buffer to card on close. */
   bool_t writing;
-  /** Slot index stored at open time so Close can derive the filename. */
-  uint8_t slot;
 } savestreamdolphin_t;
 
 /**
- * Opens a memory card slot for reading by loading its sector into buffer.
+ * Opens the consolidated memory card file for reading by loading its
+ * sector into buffer.
  *
  * @param p     Stream to initialize.
  * @param found Set to true if the file exists, false if it does not.
- * @param slot  Save slot index.
  * @return An error if reading the card fails for a reason other than
  *         missing file.
  */
-errorret_t saveStreamOpenReadDolphin(
-  savestreamdolphin_t *p, bool_t *found, const uint8_t slot
-);
+errorret_t saveStreamOpenReadDolphin(savestreamdolphin_t *p, bool_t *found);
 
 /**
- * Opens a memory card slot for writing by zeroing the sector buffer.
- * The buffer is flushed to the card when savestreamCloseDolphin is called.
+ * Opens the consolidated memory card file for writing by zeroing the
+ * sector buffer. The buffer is flushed to the card when
+ * saveStreamCloseDolphin is called.
  *
- * @param p    Stream to initialize.
- * @param slot Save slot index.
+ * @param p Stream to initialize.
  * @return An error if initialization fails.
  */
-errorret_t saveStreamOpenWriteDolphin(
-  savestreamdolphin_t *p, const uint8_t slot
-);
+errorret_t saveStreamOpenWriteDolphin(savestreamdolphin_t *p);
 
 /**
  * Flushes the sector buffer to the memory card (write mode only) and
@@ -89,3 +83,12 @@ errorret_t saveStreamWriteBytesDolphin(
  * @return An error if pos is out of range.
  */
 errorret_t saveStreamSeekDolphin(savestreamdolphin_t *p, const size_t pos);
+
+/**
+ * Gets the current read/write position within the sector buffer.
+ *
+ * @param p   Active stream.
+ * @param out Receives the current position.
+ * @return An error - always succeeds, matches saveStreamTellImpl's shape.
+ */
+errorret_t saveStreamTellDolphin(savestreamdolphin_t *p, size_t *out);