feat(series3): decode sensor self-check waveforms from the binary

The Blastware Event Report draws a "Sensor Check" strip on the right of the
waveform panel — the little traces the unit records when it pulses each sensor
before monitoring. Those live in the series-3 binary's trailing block, after
the main waveform record-chain and the per-channel calibration records, as four
length-prefixed records tagged 0x3c-0x3f (Tran/Vert/Long geophone ring-downs +
MicL pulse train). Reverse-engineered against 7 BE12844 oracle events.

New minimateplus/sensor_check.py: decode_sensor_check(raw) locates the record
chain (validated by walking the ids 0x3c->0x3f via their length prefixes) and
decodes each record's delta stream (payload[20:len-8]) with the same 10/20/30/00
delta-block tags as the main waveform codec, from an anchor of 0. Returns
{Tran,Vert,Long,MicL: [samples]} in raw 16-count units, or {} when absent.

Validated: mic pulse-train zero-crossing frequency = 20.1 Hz (exact match to
BW's mic Channel Test freq); geophone ring-downs are consistent ~-990 raw
deflections that damp to a ~-310 settle across all 7 events (a fixed
calibration pulse, so near-identical every run).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YDXjZCr4RqT2U3QvMDhgzf

tests/test_sensor_check.py

@@ -0,0 +1,67 @@
+"""Blastware sensor self-check waveform decode (minimateplus.sensor_check).
+
+Reverse-engineered 2026-09-15 against 7 BE12844 (MiniMate Plus) oracle events.
+After the main waveform record-chain and the trailing metadata / per-channel
+calibration records, a series-3 binary carries four length-prefixed records
+tagged 0x3c-0x3f: the sensor self-check traces the unit records when it pulses
+each sensor before monitoring (Blastware draws these as the little waveforms in
+the "Sensor Check" strip on the right of the Event Report).
+
+  * 0x3c / 0x3d / 0x3e = Tran / Vert / Long geophone ring-downs.
+  * 0x3f            = MicL, a pulse train at the mic self-test frequency.
+
+The self-check injects a fixed pulse, so the response is near-identical across
+events — asserted here as an invariant shape (damped one-sided ring-down for
+the geophones, a multi-pulse train for the mic).
+"""
+from pathlib import Path
+
+import numpy as np
+
+from minimateplus.sensor_check import decode_sensor_check
+
+FIXDIR = Path(__file__).parent / "fixtures" / "fft-oracle-2026-09-14"
+EVENTS = sorted(p.name for p in FIXDIR.iterdir())   # 7 BE12844 event binaries
+
+
+def _decode(name):
+    return decode_sensor_check((FIXDIR / name).read_bytes())
+
+
+def test_all_four_channels_present():
+    for name in EVENTS:
+        sc = _decode(name)
+        assert set(sc) == {"Tran", "Vert", "Long", "MicL"}, name
+
+
+def test_geo_channels_are_damped_ringdowns():
+    # Each geophone self-check is a large one-sided deflection (~-990 raw) that
+    # rings back and damps toward a settled value well above the trough.
+    for name in EVENTS:
+        sc = _decode(name)
+        for ch in ("Tran", "Vert", "Long"):
+            tr = np.asarray(sc[ch], dtype=float)
+            assert 240 <= len(tr) <= 260, f"{name}:{ch} n={len(tr)}"
+            assert abs(tr[:3].mean()) < 50, f"{name}:{ch} starts off-baseline"
+            assert tr.min() < -800, f"{name}:{ch} min {tr.min()}"
+            assert tr.max() < 60, f"{name}:{ch} unexpected positive swing {tr.max()}"
+            # damped: settles between the trough and zero, well above the trough
+            assert tr.min() < tr[-1] < 0, f"{name}:{ch} end {tr[-1]} not between trough and 0"
+            assert abs(tr[-1]) < 0.6 * abs(tr.min()), f"{name}:{ch} not damped, end {tr[-1]}"
+
+
+def test_mic_channel_is_a_pulse_train():
+    for name in EVENTS:
+        tr = np.asarray(_decode(name)["MicL"], dtype=float)
+        assert 235 <= len(tr) <= 255, f"{name} mic n={len(tr)}"
+        # larger dynamic range than the geo ring-down, and swings both ways
+        assert tr.min() < -1500, f"{name} mic min {tr.min()}"
+        assert tr.max() > 100, f"{name} mic max {tr.max()}"
+        # multiple pulses: several deep local minima
+        deep = (tr[1:-1] < tr[:-2]) & (tr[1:-1] < tr[2:]) & (tr[1:-1] < -800)
+        assert int(deep.sum()) >= 4, f"{name} mic pulses {int(deep.sum())}"
+
+
+def test_returns_empty_when_no_sensor_check_block():
+    assert decode_sensor_check(b"not a blastware file") == {}
+    assert decode_sensor_check(b"") == {}