build_messages_per_message/3 was slicing the wgrib2 -lola bin output as
a packed float32 stream, ignoring the 4-byte record-length header + 4-byte
trailer that Fortran unformatted records prepend/append around each
message's data block. First value of message 0 came back as ~2.24e-44
(little-endian float32 of the int-16 record length), and every subsequent
message was shifted by 8 bytes so values spilled into neighbouring cells.
Fix: propagate the correct offset math already used by parse_lola_binary/3
(record_overhead = 8, stride = bytes_per_message + 8, data_offset =
msg_idx * stride + 4).
Flipped the characterization test from metadata-only to per-cell physical
asserts (200 K < TMP, DPT < 340 K; DPT <= TMP) and a 0.01 K cross-check
against extract_grid/3 output on the same fixture + bbox. Latent bug —
no production callers of extract_grid_messages*.
Graham McIntire
221d5516bc