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>

scripts/backfill_sidecars.py

@@ -307,21 +307,15 @@ def main(argv=None) -> int:
                 #     (sha mismatch / tool_version too old).  The .h5 and
                 #     the sidecar are both derived from the same decoder
                 #     output, so if the sidecar is stale, so is the .h5.
-                #     This is the path that recovers from the broken-
-                #     int16-LE codec era — bumping TOOL_VERSION to 0.20.0+
-                #     marks every pre-codec sidecar stale, which now
-                #     correctly cascades to .h5 regeneration too.
                 #
-                # Skip the .h5 write when the decoder couldn't produce
-                # samples — this is the histogram-mode case today
-                # (waveform_codec.decode_waveform_v2 only handles the
-                # waveform-mode body format per §7.6.1; the histogram
-                # codec at §7.6.2 is documented but not yet implemented).
-                # Without this check we'd replace the existing (broken
-                # int16-LE) histogram .h5 with an empty one, which is
-                # arguably worse for any consumer expecting non-empty
-                # sample arrays.  When the histogram codec lands, this
-                # check can come out.
+                # Both waveform and histogram bodies now decode to real
+                # samples via event_file_io.read_blastware_file → either
+                # waveform_codec.decode_waveform_v2 or histogram_codec.
+                # decode_histogram_body.  If samples are still empty after
+                # both codecs run, it's a genuine "we can't decode this
+                # file" case (truncated, malformed, or unknown mode);
+                # skip the .h5 write so we don't replace whatever's
+                # there with an empty placeholder.
                 has_samples = bool(
                     ev.raw_samples and any(
                         ev.raw_samples.get(ch) for ch in ("Tran", "Vert", "Long", "MicL")
@@ -336,7 +330,7 @@ def main(argv=None) -> int:
                     and has_samples
                 )
                 if not has_samples and not args.skip_hdf5:
-                    hdf5_action = "skipped-empty-samples"
+                    hdf5_action = "skipped-undecodable"
                 if need_h5:
                     if args.dry_run:
                         hdf5_action = "would (re)write"