fix(sfm): read the serial family prefix from the file, enabling BlastMates

The BW filename encodes only the serial NUMBER — `<letter><3 digits>`, so
`L895…` is 10895 and nothing more. The two-letter family prefix is not in it:
"BE" is a MiniMate Plus, "BA" a BlastMate. Both are Series III and their
files are byte-identical in every way that matters — all 1,493 BlastMate
binaries in the DL2 archive decode through the existing codec at 100%, same
four channels — so the serial string was the only thing standing between SFM
and BlastMate support.

Two sites synthesised the prefix and got it wrong:

- waveform_store `_serial_from_bw_filename` returned f"BE{num}" on import, so
  a BlastMate event was filed under a unit that does not exist, silently, and
  Terra-View read it straight through. Split into
  `_serial_number_from_bw_filename` (the number, which the filename really
  does carry) and a new `_serial_from_bw_bytes` that reads the serial out of
  the body and accepts it only when its numeric part agrees with the
  filename. save_imported_bw now prefers hint -> body -> filename guess.
  Verified against real archive bytes for BA9229, BA10060, BA10895, BA15957
  and BE9558/BE11529/BE18003.

- client `_decode_0a_partial_header` searched for a literal b"BE" in the
  monitor-log partial record. On a BlastMate that returns -1 and skips the
  whole block, so the geo threshold went missing along with the serial. Now
  matches any two-letter prefix, and requires the NUL terminator — stricter
  than the bare two-byte search it replaces.

Nothing to migrate: no BlastMate events are in prod. The archive's BA units
last recorded 2018-10 (BA9229, BA15957), 2023-08 (BA10895) and 2023-11
(BA10060), and the prod backfill only reaches back to ~May 2025.

21 tests. Suite: 309 passed, same 16 pre-existing failures as at HEAD
(15 missing ASCII fixtures + one peak_values assertion, all untouched here).

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HgTe8CamXAHcAmaQ6QNcog

minimateplus/client.py

@@ -30,6 +30,7 @@ from __future__ import annotations
 
 import datetime
 import logging
+import re
 import struct
 from typing import Optional
 
@@ -2532,10 +2533,17 @@ def _decode_0a_partial_header(raw_data: bytes, index: int, key4: bytes) -> Optio
         ts2 = try_ts(raw_data[ts1_end + 1:ts1_end + 1 + ts_size])
 
     # Extract serial and geo threshold from "BE11529\0" and "Geo: X.XXX in/s\0".
+    #
+    # Match any two-letter family prefix, not a literal "BE" — a BlastMate
+    # reports "BA10895", and the old `find(b"BE")` returned -1 on one.  That
+    # skipped this whole block, so the geo threshold went missing along with
+    # the serial.  Requiring the NUL terminator in the pattern also makes the
+    # match stricter than the bare two-byte search it replaces.
     serial: Optional[str] = None
     geo_ips: Optional[float] = None
 
-    serial_pos = raw_data.find(b"BE")
+    serial_match = re.search(rb"[A-Z]{2}\d{3,6}(?=\x00)", raw_data)
+    serial_pos = serial_match.start() if serial_match else -1
     if serial_pos >= 0:
         # Read null-terminated serial starting at serial_pos.
         null_pos = raw_data.find(b"\x00", serial_pos)