Fix GameCube memory card slot B detection and free-space check

CARD_Mount() alone reliably reports CARD_ERROR_NOCARD for a card physically
present in slot B - unlike slot A, which the IPL polls automatically at
boot, slot B needs an explicit CARD_Probe() first (the standard pattern in
every official devkitPro CARD sample). Also fixes
saveDeviceDolphinCardHasFreeSpace() treating CARD_GetDirectory()'s
CARD_ERROR_NOFILE (a totally empty card, not an actual error) as a hard
failure, which made any card with zero existing save files - slot B's in
this case, since slot A already had leftover test data on it - get
reported as full.

Confirmed via Dolphin with slot A disabled/slot B set to a memory card:
previously reported "no save device found", now correctly detects it.

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

src/duskdolphin/save/savedevicedolphincard.c

@@ -52,6 +52,21 @@ void saveDeviceDolphinCardCheckAvailability(savedevice_t *device) {
     return saveDeviceFireCallback(device);
   }
 
+  // Slot A gets polled automatically by the IPL at boot, but slot B does
+  // not - CARD_Mount() alone reliably reports CARD_ERROR_NOCARD for a
+  // present slot B card unless CARD_Probe() is used to explicitly detect
+  // it first (matches the probe-then-mount pattern in every official
+  // devkitPro CARD sample, not something slot A happens to need too).
+  do {
+    result = CARD_Probe(platform->channel);
+  } while(result == CARD_ERROR_BUSY);
+
+  if(result < 0) {
+    device->state = SAVE_DEVICE_STATE_UNAVAILABLE;
+    device->reasonKey = saveDeviceDolphinCardErrorReasonKey(result);
+    return saveDeviceFireCallback(device);
+  }
+
   do {
     result = CARD_Mount(platform->channel, platform->cardBuffer, NULL);
   } while(result == CARD_ERROR_BUSY);
@@ -59,20 +74,7 @@ void saveDeviceDolphinCardCheckAvailability(savedevice_t *device) {
   if(result < 0) {
     platform->mounted = false;
     device->state = SAVE_DEVICE_STATE_UNAVAILABLE;
-    switch(result) {
+    device->reasonKey = saveDeviceDolphinCardErrorReasonKey(result);
-      case CARD_ERROR_NOCARD:
-        device->reasonKey = "save.dolphin.no_card";
-        break;
-      case CARD_ERROR_WRONGDEVICE:
-        device->reasonKey = "save.dolphin.wrong_device";
-        break;
-      case CARD_ERROR_BROKEN:
-        device->reasonKey = "save.dolphin.broken";
-        break;
-      default:
-        device->reasonKey = "save.dolphin.mount_failed";
-        break;
-    }
     return saveDeviceFireCallback(device);
   }
   platform->mounted = true;
@@ -88,6 +90,15 @@ void saveDeviceDolphinCardCheckAvailability(savedevice_t *device) {
   saveDeviceFireCallback(device);
 }
 
+const char_t *saveDeviceDolphinCardErrorReasonKey(const int32_t result) {
+  switch(result) {
+    case CARD_ERROR_NOCARD: return "save.dolphin.no_card";
+    case CARD_ERROR_WRONGDEVICE: return "save.dolphin.wrong_device";
+    case CARD_ERROR_BROKEN: return "save.dolphin.broken";
+    default: return "save.dolphin.mount_failed";
+  }
+}
+
 bool_t saveDeviceDolphinCardHasFreeSpace(const int32_t channel) {
   uint16_t blockCount = 0;
   if(CARD_GetBlockCount(channel, &blockCount) < 0) return false;
@@ -101,7 +112,12 @@ bool_t saveDeviceDolphinCardHasFreeSpace(const int32_t channel) {
   // game itself has already written.
   card_dir entries[CARD_MAXFILES];
   int32_t count = 0;
-  if(CARD_GetDirectory(channel, entries, &count, true) < 0) return false;
+  int32_t dirResult = CARD_GetDirectory(channel, entries, &count, true);
+  // A totally empty card (no files at all yet) reports CARD_ERROR_NOFILE
+  // here rather than success with count 0 - not a real error, and by far
+  // the most common state for a card nobody has saved to yet.
+  if(dirResult < 0 && dirResult != CARD_ERROR_NOFILE) return false;
+  if(dirResult == CARD_ERROR_NOFILE) count = 0;
   if(count >= CARD_MAXFILES) return false;// No free file entry left either.
 
   uint32_t usedBlocks = 0;

src/duskdolphin/save/savedevicedolphincard.h

@@ -53,6 +53,15 @@ errorret_t saveDeviceDolphinCardUpdate(savedevice_t *device);
  */
 void saveDeviceDolphinCardCheckAvailability(savedevice_t *device);
 
+/**
+ * Maps a libogc CARD_ERROR_* result (from CARD_Probe or CARD_Mount) to the
+ * matching reasonKey.
+ *
+ * @param result The card_errors result code to map.
+ * @return The matching reasonKey, or a generic fallback if unrecognized.
+ */
+const char_t *saveDeviceDolphinCardErrorReasonKey(const int32_t result);
+
 /**
  * Returns whether the mounted memory card in the given slot has at least
  * one block of free space and one free file entry remaining, by summing