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/duskpsp/save/savepsp.c

@@ -7,7 +7,18 @@
 
 #include "save/save.h"
 
+void savePSPEnsureBaseDirs(void) {
+  sceIoMkdir(SAVE_PSP_BASE_DIR, 0777);
+  sceIoMkdir(SAVE_PSP_SAVEDATA_DIR, 0777);
+}
+
 errorret_t saveInitPSP(void) {
+  SceIoStat stat;
+  if(sceIoGetstat(SAVE_PSP_ROOT, &stat) < 0) {
+    errorThrow("No memory stick detected");
+  }
+
+  savePSPEnsureBaseDirs();
   errorOk();
 }
 
@@ -40,6 +51,7 @@ errorret_t saveLoadPSP(const uint8_t slot, savefile_t *file) {
 }
 
 errorret_t saveWritePSP(const uint8_t slot, const savefile_t *file) {
+  savePSPEnsureBaseDirs();
   char_t dir[SAVE_PSP_PATH_MAX];
   snprintf(dir, SAVE_PSP_PATH_MAX, SAVE_PSP_DIR_FORMAT,
     SAVE_PSP_TITLE_ID, (uint32_t)slot

src/duskpsp/save/savepsp.h

@@ -11,6 +11,9 @@
 #include <pspiofilemgr.h>
 
 #define SAVE_PSP_PATH_MAX 256
+#define SAVE_PSP_ROOT "ms0:/"
+#define SAVE_PSP_BASE_DIR "ms0:/PSP"
+#define SAVE_PSP_SAVEDATA_DIR "ms0:/PSP/SAVEDATA"
 #define SAVE_PSP_FILE_FORMAT "ms0:/PSP/SAVEDATA/%s%02u/save.dat"
 #define SAVE_PSP_DIR_FORMAT "ms0:/PSP/SAVEDATA/%s%02u"
 
@@ -23,9 +26,14 @@ typedef struct {
 } savepsp_t;
 
 /**
- * Initializes the save system on PSP.
+ * Initializes the save system on PSP. Confirms the memory stick is
+ * actually reachable (sceIoGetstat on SAVE_PSP_ROOT) rather than assuming
+ * so, since raw sceIo calls otherwise only fail once something tries to
+ * touch the filesystem - and ensures the PSP/SAVEDATA directory tree
+ * exists (SAVE_PSP_BASE_DIR then SAVE_PSP_SAVEDATA_DIR, since sceIoMkdir
+ * only creates one level at a time).
  *
- * @return An error code if initialization fails.
+ * @return An error code if no memory stick is reachable.
  */
 errorret_t saveInitPSP(void);
 
@@ -61,3 +69,12 @@ errorret_t saveWritePSP(const uint8_t slot, const savefile_t *file);
  * @return An error code if the delete fails.
  */
 errorret_t saveDeletePSP(const uint8_t slot);
+
+/**
+ * Ensures SAVE_PSP_BASE_DIR and SAVE_PSP_SAVEDATA_DIR both exist, creating
+ * whichever are missing. sceIoMkdir only creates one directory level at a
+ * time, so this must run before creating any per-slot save directory
+ * beneath SAVE_PSP_SAVEDATA_DIR. Safe to call repeatedly - an
+ * already-exists result is not an error.
+ */
+void savePSPEnsureBaseDirs(void);

src/duskpsp/save/savestreampsp.c

@@ -22,6 +22,7 @@ errorret_t saveStreamOpenReadPSP(
 }
 
 errorret_t saveStreamOpenWritePSP(savestreampsp_t *p, const uint8_t slot) {
+  savePSPEnsureBaseDirs();
   char_t dir[SAVE_PSP_PATH_MAX];
   snprintf(dir, SAVE_PSP_PATH_MAX, SAVE_PSP_DIR_FORMAT,
     SAVE_PSP_TITLE_ID, (uint32_t)slot