feat: implement Dmod_ReadDirEx in dmvfs SAL layer

[Copilot]

Dmod_ReadDirEx was declared in dmod_sal.h and had a weak POSIX fallback in DMOD, but dmvfs never provided its own override — leaving the interface unsupported when using the dmvfs SAL.

Changes

• src/dmod_sal.c — Adds _ReadDirEx implementation that:
  • Calls dmvfs_readdir to retrieve the next dmfsi_dir_entry_t
  • Maps DMFSI_ATTR_DIRECTORY → Dmod_DirEntryType_Dir, otherwise Dmod_DirEntryType_File
  • Returns a pointer to a file-scoped static Dmod_DirEntry_t (same lifetime contract as _ReadDir)
  • Reuses g_last_dir_entry_name buffer shared with _ReadDir
  • Promotes g_last_dir_entry to file scope alongside g_last_dir_entry_name, with unified thread-safety docs covering both functions

void* dir = Dmod_OpenDir("/mnt/");
const Dmod_DirEntry_t* entry;
while ((entry = Dmod_ReadDirEx(dir)) != NULL) {
    printf("%s (%s)\n", entry->name,
           entry->type == Dmod_DirEntryType_Dir ? "dir" : "file");
}
Dmod_CloseDir(dir);