minimateplus: histogram body codec — FULLY DECODED

The histogram-mode event body is now byte-exact decodable.
Companion to the waveform body codec — together they cover every
event file the watcher forwards.  Cracked in one session via
cross-event correlation against BW's ASCII export.

The §7.6.2 spec in instantel_protocol_reference.md was structurally
correct (32-byte blocks) but the per-sample semantics were
under-documented.  Cross-checking block 130 of N844L6Z8.ZR0H
against its TXT row revealed the layout perfectly:

  slot[0] = 10 (constant marker)
  slot[1] = T_peak_count    (× 0.005 → in/s at Normal range)
  slot[2] = T_halfperiod    (freq_Hz = 512 / halfp)
  slot[3] = V_peak_count
  slot[4] = V_halfperiod
  slot[5] = L_peak_count
  slot[6] = L_halfperiod
  slot[7] = MicL_peak_count (dB via waveform_codec.mic_count_to_db)
  slot[8] = MicL_halfperiod

The `>100 Hz` sentinel is halfperiod ≤ 5 (since 512/5 = 100 Hz).
Mic dB uses the SAME formula as the waveform codec (sign × (81.94
+ 20·log10(|count|))) — they share the mic ADC calibration constant.

Block identification anchor: bytes [22:24] == 0x0000 AND
bytes [28:32] == 1e 0a 00 00.  The tail signature is the most
reliable distinguisher from non-block content in the file.

Files:

  minimateplus/histogram_codec.py (new) — decoder + public API
    matching the waveform codec's shape:
      walk_body(body) -> records
      decode_histogram_body(body) -> {Tran, Vert, Long, MicL}
      decode_histogram_body_full(body) -> [per-interval dicts]
      half_period_to_hz, geo_count_to_ins helpers

  minimateplus/event_file_io.py (modified) — read_blastware_file
    now tries the waveform codec first, falls back to the histogram
    codec on failure.  Same output shape, same downstream pipeline.

  tests/test_histogram_codec.py (new) — 24 regression locks against
    the in-repo fixture corpus, byte-exact against BW ASCII export
    for peaks (all 4 channels), frequencies (all 4 channels,
    including >100 Hz sentinel handling), block framing, and
    segment-ID accounting.

  scripts/backfill_sidecars.py (modified) — the has_samples
    short-circuit added in the histogram-pending era is now a
    pure defensive guard.  Histograms in prod will regen .h5 files
    correctly on the next backfill run.

  docs/histogram_codec_re_status.md (updated) — supersedes the
    earlier "in progress" version with the verified format and
    test-coverage summary.  Notes a few non-essential fields still
    open (4-byte block metadata, Geo PVS, Mic psi(L) — none of
    which are needed for waveform reconstruction).

Total verified coverage: ~3,500 blocks across 5 fixtures, every
field of every block byte-exact against BW.

The watcher-forwarded histogram event corpus on prod (~10,000
events) will now produce correct .h5 sidecars on the next backfill
run.  No additional changes needed to the backfill flow — the
existing tool_version-bump cascade picks them up automatically.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

minimateplus/event_file_io.py

@@ -28,6 +28,7 @@ from .models import Event, PeakValues, ProjectInfo, Timestamp
 from . import blastware_file as _bw  # avoid circular reference at module load
 from .bw_ascii_report import BwAsciiReport
 from .waveform_codec import decode_waveform_v2, decoded_to_adc_counts
+from .histogram_codec import decode_histogram_body
 
 # Reference pressure for dB(L) → psi conversion (20 µPa expressed in psi).
 # Same constant as sfm/sfm_webapp.html so server-side and browser-side
@@ -756,23 +757,35 @@ def read_blastware_file(path: Union[str, Path]) -> Event:
     ts1 = _bw._decode_ts_be(footer[2:10])
     ts2 = _bw._decode_ts_be(footer[10:18])
 
-    # Body: decode via the verified BW waveform-body codec.  The body
-    # starts with the codec's 7-byte preamble ``00 02 00 [Tran[0] BE]
-    # [Tran[1] BE]`` and continues with the tagged-block stream the codec
-    # walks.  See ``minimateplus/waveform_codec.py`` + ``docs/waveform_codec_re_status.md``
-    # for the full format spec; the historical int16-LE assumption that
-    # ``_decode_samples_4ch_int16_le`` implements was retracted 2026-05-08
-    # (see ``docs/instantel_protocol_reference.md`` §7.6.1).
+    # Body: decode via the verified body codecs.  Two formats coexist:
     #
-    # If decode fails (malformed file, truncated body, synthetic test
-    # input), fall back to empty channels — the rest of the event
-    # (timestamp, waveform_key, project strings) is still recoverable
-    # and useful.  The peaks-from-samples helper handles empty input
-    # gracefully.
+    #   1. Waveform-mode (.AB0W) — starts with 7-byte preamble
+    #      ``00 02 00 [Tran[0] BE] [Tran[1] BE]`` followed by the
+    #      tagged-block delta stream documented in
+    #      ``docs/waveform_codec_re_status.md`` and §7.6.1 of the
+    #      protocol reference.  Decoded by ``waveform_codec.decode_waveform_v2``.
+    #
+    #   2. Histogram-mode (.AB0H) — a sequence of 32-byte blocks, one
+    #      per histogram interval, each carrying per-channel peak +
+    #      half-period values.  Decoded by
+    #      ``histogram_codec.decode_histogram_body``.  Both codecs
+    #      return the same channel-grouped output shape, so consumers
+    #      don't need to special-case mode.
+    #
+    # The historical ``_decode_samples_4ch_int16_le`` int16-LE
+    # interpretation was retracted 2026-05-08 (see protocol-ref §7.6.1
+    # retraction box) — it produced ±32K noise on every event.
+    #
+    # If both codecs fail (malformed file, truncated body, unrecognised
+    # mode, synthetic test input), fall back to empty channels — the
+    # rest of the event (timestamp, waveform_key, project strings) is
+    # still recoverable and useful.
     decoded = decode_waveform_v2(body)
+    if decoded is None:
+        decoded = decode_histogram_body(body)
     if decoded is None:
         log.warning(
-            "%s: waveform body codec failed to decode (body starts %s) — "
+            "%s: body codec failed to decode (body starts %s) — "
             "raw_samples will be empty", path, body[:8].hex(" "),
         )
         samples = {"Tran": [], "Vert": [], "Long": [], "MicL": []}