Re-enable save system, fix header/version stamping, handle missing media

- Fixed the actual reason saving never worked on any platform: saveWrite()
  never stamped file->header/file->version before serializing, so every
  written save file had a zeroed magic header and failed its own
  validation on the next load. Confirmed via a manual write/load round
  trip that this alone fully explains "saving doesn't work."
- Re-enabled saveInit()/saveDispose() in engine.c (previously commented out
  under "Temporarily disable save code").
- Added SAVE.available + saveIsAvailable(), refreshed by every real
  save/load/delete attempt. saveInit() no longer treats an unreachable
  save medium as fatal to booting - it logs and continues, since a missing
  memory card/stick shouldn't prevent playing.
- Hardened PSP's saveInitPSP() to actually detect a missing memory stick
  (sceIoGetstat on ms0:/) instead of assuming success, and fixed
  single-level sceIoMkdir to build the full PSP/SAVEDATA directory chain.
- Added busy-retry (CARD_ERROR_BUSY) and a not-mounted guard to Dolphin's
  live savestreamdolphin.c path, extending the same handling already
  backported into savedolphin.c.
- Added a "Save" entry to the game menu wired to saveWrite(0), showing a
  clear message on success, on failure, and when saveIsAvailable() is false.

src/duskdolphin/save/savestreamdolphin.c

@@ -19,12 +19,20 @@ static void _saveStreamGetFileName(
 errorret_t saveStreamOpenReadDolphin(
   savestreamdolphin_t *p, bool_t *found, const uint8_t slot
 ) {
+  if(!SAVE.platform.mounted) {
+    *found = false;
+    errorThrow("No memory card mounted");
+  }
+
   char_t fileName[SAVE_DOLPHIN_FILE_NAME_MAX];
   _saveStreamGetFileName(fileName, SAVE_DOLPHIN_FILE_NAME_MAX, slot);
 
-  int32_t result = CARD_Open(
-    SAVE_DOLPHIN_CHANNEL, fileName, &p->cardFile
-  );
+  int32_t result;
+  do {
+    result = CARD_Open(
+      SAVE_DOLPHIN_CHANNEL, fileName, &p->cardFile
+    );
+  } while(result == CARD_ERROR_BUSY);
   if(result == CARD_ERROR_NOFILE) {
     *found = false;
     p->position = 0;
@@ -33,17 +41,19 @@ errorret_t saveStreamOpenReadDolphin(
   }
   if(result < 0) {
     *found = false;
-    errorThrow("Failed to open memory card file for slot %u (error %d)",
-      (uint32_t)slot, result
+    errorThrow("Failed to open memory card file for slot %u: %s (%d)",
+      (uint32_t)slot, saveCardErrorStringDolphin(result), result
     );
   }
 
-  result = CARD_Read(&p->cardFile, p->buffer, SAVE_DOLPHIN_SECTOR_SIZE, 0);
+  do {
+    result = CARD_Read(&p->cardFile, p->buffer, SAVE_DOLPHIN_SECTOR_SIZE, 0);
+  } while(result == CARD_ERROR_BUSY);
   CARD_Close(&p->cardFile);
   if(result < 0) {
     *found = false;
-    errorThrow("Failed to read memory card data for slot %u (error %d)",
-      (uint32_t)slot, result
+    errorThrow("Failed to read memory card data for slot %u: %s (%d)",
+      (uint32_t)slot, saveCardErrorStringDolphin(result), result
     );
   }
 
@@ -57,6 +67,8 @@ errorret_t saveStreamOpenReadDolphin(
 errorret_t saveStreamOpenWriteDolphin(
   savestreamdolphin_t *p, const uint8_t slot
 ) {
+  if(!SAVE.platform.mounted) errorThrow("No memory card mounted");
+
   memoryZero(p->buffer, SAVE_DOLPHIN_SECTOR_SIZE);
   p->position = 0;
   p->writing = true;
@@ -70,14 +82,21 @@ void saveStreamCloseDolphin(savestreamdolphin_t *p) {
   char_t fileName[SAVE_DOLPHIN_FILE_NAME_MAX];
   _saveStreamGetFileName(fileName, SAVE_DOLPHIN_FILE_NAME_MAX, p->slot);
 
-  int32_t result = CARD_Open(SAVE_DOLPHIN_CHANNEL, fileName, &p->cardFile);
+  int32_t result;
+  do {
+    result = CARD_Open(SAVE_DOLPHIN_CHANNEL, fileName, &p->cardFile);
+  } while(result == CARD_ERROR_BUSY);
   if(result == CARD_ERROR_NOFILE) {
-    CARD_Create(
-      SAVE_DOLPHIN_CHANNEL, fileName, SAVE_DOLPHIN_SECTOR_SIZE, &p->cardFile
-    );
+    do {
+      result = CARD_Create(
+        SAVE_DOLPHIN_CHANNEL, fileName, SAVE_DOLPHIN_SECTOR_SIZE, &p->cardFile
+      );
+    } while(result == CARD_ERROR_BUSY);
   }
 
-  CARD_Write(&p->cardFile, p->buffer, SAVE_DOLPHIN_SECTOR_SIZE, 0);
+  do {
+    result = CARD_Write(&p->cardFile, p->buffer, SAVE_DOLPHIN_SECTOR_SIZE, 0);
+  } while(result == CARD_ERROR_BUSY);
   CARD_Close(&p->cardFile);
 }