Fix T93858: Zstd-compressed .blend files from external tools aren't recognized

The issue here was that after the seek table check, the underlying file wasn't
rewound to the start, so the code that checks for the BLENDER header
immediately reaches EOF and fails.

Since Blender always writes files with a seek table, this bug isn't triggered
by files saved in Blender itself. However, files compressed in external tools
generally don't have a seek table.

source/blender/blenlib/intern/filereader_zstd.c

@@ -331,5 +331,8 @@ FileReader *BLI_filereader_new_zstd(FileReader *base)
   }
   zstd->reader.close = zstd_close;
 
+  /* Rewind after the seek table check so that zstd_read starts at the file's start. */
+  zstd->base->seek(zstd->base, 0, SEEK_SET);
+
   return (FileReader *)zstd;
 }