derive: restore the header read

scripts/derive-protected-experts.py

@@ -37,6 +37,7 @@ NAME_RE = re.compile(r"^gram\.r(\d)\.blk\.(\d+)\.ffn_down_exps\.(\d+)\.bin$")
 def read_gram(path):
     """(k, rank, rows, trace) from a RADGRAM2 file (full matrix on disk)."""
     with open(path, "rb") as f:
+        hdr = f.read(32)
         magic, k, rank, rows, domain = struct.unpack("<8sIIQI", hdr[:28])
         if magic != b"RADGRAM2":
             raise SystemExit(f"not a RADGRAM2 file: {path}")