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/saveplatform.h

@@ -7,24 +7,21 @@
 
 #pragma once
 #include "save/savelinux.h"
-#include "save/savestreamlinux.h"
 
 typedef savelinux_t saveplatform_t;
-typedef savestreamlinux_t saveplatformstream_t;
+// Linux fully overrides every save operation with JSON I/O (see
+// savelinux.c/savejsonlinux.c) - nothing generic ever opens a
+// savestream_t here, so this only needs to exist for that shared type to
+// compile.
+typedef struct {
+  uint8_t reserved;
+} saveplatformstream_t;
 
 #define saveInitPlatform saveInitLinux
 #define saveDisposePlatform saveDisposeLinux
-#define saveDeletePlatform saveDeleteLinux
 
-#define saveStreamOpenReadPlatform(stream, slot) \
-  saveStreamOpenReadLinux(&(stream)->platform, &(stream)->found, slot)
-#define saveStreamOpenWritePlatform(stream, slot) \
-  saveStreamOpenWriteLinux(&(stream)->platform, slot)
-#define saveStreamClosePlatform(stream) \
-  saveStreamCloseLinux(&(stream)->platform)
-#define saveStreamReadBytesPlatform(stream, buf, len) \
-  saveStreamReadBytesLinux(&(stream)->platform, buf, len)
-#define saveStreamWriteBytesPlatform(stream, buf, len) \
-  saveStreamWriteBytesLinux(&(stream)->platform, buf, len)
-#define saveStreamSeekPlatform(stream, pos) \
-  saveStreamSeekLinux(&(stream)->platform, pos)
+#define saveSlotDeletePlatform saveDeleteSlotLinux
+#define saveSlotLoadPlatform saveSlotLoadLinux
+#define saveSlotWritePlatform saveSlotWriteLinux
+#define saveMetaLoadPlatform saveMetaLoadLinux
+#define saveMetaWritePlatform saveMetaWriteLinux