Add compressed combined save format for PSP and Dolphin

Memory sticks/cards don't suit N+1 separate save files the way Linux's
filesystem does, so add an opt-in SAVE_DEVICE_DATA_RAW mode: settings and
all save slots get serialized to JSON, concatenated, zlib-compressed, and
framed with a magic/version/checksum/generation header, then read/written
as a single blob through one combined platform hook instead of four.

- PSP and Dolphin's SD/NAND backends write via a temp file + atomic rename,
  so a crash mid-write can never leave a half-written save behind.
- GameCube memory cards have no rename or resize primitive, so they
  ping-pong between two fixed files instead, picking whichever is valid and
  has the higher generation counter on load.
- Linux is untouched (still four separate JSON files); saveslot.h/
  savesettings.h drop their now-unnecessary pack(1) now that nothing
  persists them as raw bytes.

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

src/duskdolphin/save/savedevicedolphincard.h

@@ -17,6 +17,20 @@
   #define SAVE_DEVICE_DOLPHIN_GAME_CODE "DUSK"
 #endif
 
+// Memory cards have no rename primitive and files can't be resized once
+// created, so a single "current" file can't be replaced atomically the way
+// the other backends do. Instead saveDeviceDolphinCardDataWrite() ping-pongs
+// between these two fixed files - every store targets whichever one is NOT
+// the currently valid+newest copy (by saveDeviceRawIsValid()'s generation
+// counter), so a crash mid-write can only ever corrupt the copy nobody
+// would load from; the other one stays intact as the fallback.
+#ifndef SAVE_DEVICE_DOLPHIN_CARD_FILENAME_0
+  #define SAVE_DEVICE_DOLPHIN_CARD_FILENAME_0 "DUSKSAVE0"
+#endif
+#ifndef SAVE_DEVICE_DOLPHIN_CARD_FILENAME_1
+  #define SAVE_DEVICE_DOLPHIN_CARD_FILENAME_1 "DUSKSAVE1"
+#endif
+
 typedef struct {
   // Set from this device's index into SAVE.devices - device 0 is
   // CARD_SLOTA, device 1 is CARD_SLOTB.
@@ -82,3 +96,38 @@ bool_t saveDeviceDolphinCardHasFreeSpace(const int32_t channel);
  * @return Error state if any.
  */
 errorret_t saveDeviceDolphinCardDispose(savedevice_t *device);
+
+/**
+ * Writes the combined save data blob out to whichever of the two fixed
+ * card files (see SAVE_DEVICE_DOLPHIN_CARD_FILENAME_0/1 above) is not
+ * currently the valid+newest copy - ping-pong in place of the atomic
+ * rename the other backends use, since the CARD API has neither a rename
+ * nor a way to resize an existing file.
+ *
+ * @param device The save device to write to.
+ * @param buffer The raw bytes to write.
+ * @param size The number of bytes to write.
+ * @return Error state if any.
+ */
+errorret_t saveDeviceDolphinCardDataWrite(
+  savedevice_t *device,
+  const uint8_t *buffer,
+  size_t size
+);
+
+/**
+ * Reads whichever of the two fixed card files is valid and has the higher
+ * generation counter. If neither file exists or validates, this is not an
+ * error - *outBuffer is set to NULL and *outSize to 0.
+ *
+ * @param device The save device to read from.
+ * @param outBuffer Receives a memoryAlign'd (32-byte) buffer the caller
+ *   must free with memoryFree, or NULL if nothing has been saved yet.
+ * @param outSize Receives the number of bytes in *outBuffer.
+ * @return Error state if any.
+ */
+errorret_t saveDeviceDolphinCardDataRead(
+  savedevice_t *device,
+  uint8_t **outBuffer,
+  size_t *outSize
+);