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/dusklinux/save/savelinux.h

@@ -7,10 +7,12 @@
 
 #pragma once
 #include "error/error.h"
-#include "save/savefile.h"
+#include "save/saveslot.h"
+#include "save/savemeta.h"
 
 #define SAVE_LINUX_PATH_MAX FILENAME_MAX
-#define SAVE_LINUX_FILE_FORMAT "%s/save_%u.dat"
+#define SAVE_LINUX_SLOT_FILE_FORMAT "%s/slot%u.json"
+#define SAVE_LINUX_META_FILE_FORMAT "%s/settings.json"
 
 #ifndef SAVE_LINUX_PATH
   #define SAVE_LINUX_PATH "./saves"
@@ -21,7 +23,8 @@ typedef struct {
 } savelinux_t;
 
 /**
- * Initializes the save system on Linux.
+ * Initializes the save system on Linux - ensures the save directory
+ * exists (shared by both save slots and meta).
  *
  * @return An error code if initialization fails.
  */
@@ -35,27 +38,43 @@ errorret_t saveInitLinux(void);
 errorret_t saveDisposeLinux(void);
 
 /**
- * Loads a save file from disk for the given slot.
+ * Loads a save slot as JSON (slotN.json) from disk, if it exists.
  *
  * @param slot The save slot index.
- * @param file Output save file data.
- * @return An error code if the load fails.
+ * @param out Output slot data.
+ * @return An error code if the slot exists but fails to parse.
  */
-errorret_t saveLoadLinux(const uint8_t slot, savefile_t *file);
+errorret_t saveSlotLoadLinux(const uint8_t slot, saveslot_t *out);
 
 /**
- * Writes a save file to disk for the given slot.
+ * Writes a save slot as JSON (slotN.json) to disk.
  *
  * @param slot The save slot index.
- * @param file Save file data to write.
+ * @param slotData Slot data to write.
  * @return An error code if the write fails.
  */
-errorret_t saveWriteLinux(const uint8_t slot, const savefile_t *file);
+errorret_t saveSlotWriteLinux(const uint8_t slot, saveslot_t *slotData);
 
 /**
- * Deletes the save file for the given slot from disk.
+ * Deletes the save slot JSON file for the given index.
  *
  * @param slot The save slot index.
  * @return An error code if the delete fails.
  */
-errorret_t saveDeleteLinux(const uint8_t slot);
+errorret_t saveDeleteSlotLinux(const uint8_t slot);
+
+/**
+ * Loads save meta as JSON (settings.json) from disk, if it exists.
+ *
+ * @param out Output meta data.
+ * @return An error code if the file exists but fails to parse.
+ */
+errorret_t saveMetaLoadLinux(savemeta_t *out);
+
+/**
+ * Writes save meta as JSON (settings.json) to disk.
+ *
+ * @param meta Meta data to write.
+ * @return An error code if the write fails.
+ */
+errorret_t saveMetaWriteLinux(savemeta_t *meta);