codec: wire decode_waveform_v2 into production; add MicL dB helper

Replaces the broken legacy int16 LE decoder in client.py with the
verified multi-channel codec.  Three changes:

1. blastware_file.extract_body_bytes(a5_frames) — new helper that
   factors out the body-reconstruction logic from write_blastware_file
   so both writers (BW binary) and decoders (sample arrays) can use
   the same canonical bytes.

2. waveform_codec.decode_a5_frames(a5_frames) — production entry point.
   Returns the raw_samples dict consumers expect (Tran/Vert/Long as
   int16 ADC counts; MicL as native ADC counts).  Internally:
     A5 frames → extract_body_bytes → decode_waveform_v2
                → decoded_to_adc_counts (geos ×16; mic pass-through)

3. waveform_codec.mic_count_to_db(count) — MicL ADC → dB(L) per BW's
   display formula:
     dB = sign(count) × (81.94 + 20 × log10(|count|))   for |count| ≥ 1
   Verified against V70 fixture: count=813 → 140.14 dB (BW PSPL 140.1).

client.py:_decode_a5_waveform is reduced to a thin wrapper that calls
decode_a5_frames and populates event.raw_samples.  Original implementation
preserved as _decode_a5_waveform_LEGACY (dead code; reference only).

Also fixed a tail-end bug in decode_waveform_v2 where trailer-section
"40 02" markers (containing ASCII serial bytes, NOT real segment headers)
were being mis-interpreted, producing 2 spurious samples per channel at
the end of each event.  Added bytes [12:14] == "02 00" validation to
reject non-header markers.

7 new pytest tests cover the new helpers and dB conversion.  Total:
71 passing (up from 64).

Known limitation (carried over from before): the walker still stops
mid-event on the loudest fixtures (SP0/SS0/SV0/event-b) at some
mid-segment edge cases not yet characterized.  Every sample reached
is decoded correctly; the walker just doesn't reach all of them.
Loud events still yield 5,000–15,000 byte-exact samples each.

minimateplus/blastware_file.py

@@ -552,6 +552,105 @@ def classify_frame(frame: S3Frame) -> str:
 
 # ── Waveform file writer ───────────────────────────────────────────────────────────
 
+def extract_body_bytes(a5_frames):
+    """Reconstruct the Blastware-file body bytes from a list of A5 frames.
+
+    Returns ``(strt, body, footer)`` where:
+
+    - ``strt`` is the 21-byte STRT record from the probe frame (or a fallback
+      record built from minimal event metadata if STRT is missing).
+    - ``body`` is the variable-length sample-data section (between STRT and
+      the 26-byte file footer).  Empty if no frames decode.
+    - ``footer`` is the 26-byte file footer.
+
+    This is the same body-construction algorithm used by :func:`write_blastware_file`
+    — refactored out so the body decoder (``waveform_codec.decode_waveform_v2``)
+    can consume the same bytes without re-implementing the frame-walking logic.
+
+    Returns ``(b"", b"", b"")`` if *a5_frames* is empty.
+    """
+    if not a5_frames:
+        return (b"", b"", b"")
+
+    # ── Extract STRT record from probe frame ─────────────────────────────────
+    w0_raw = bytes(a5_frames[0].data[7:])
+    w0_stripped = _strip_inner_frame_dles(w0_raw)
+    strt_pos_stripped = w0_stripped.find(b"STRT")
+
+    if strt_pos_stripped >= 0:
+        strt = bytes(w0_stripped[strt_pos_stripped : strt_pos_stripped + 21])
+
+        # Walk raw bytes to find the raw-domain end of the STRT (= body start).
+        target_stripped = strt_pos_stripped + 21
+        stripped_so_far = 0
+        raw_i = 0
+        while stripped_so_far < target_stripped and raw_i < len(w0_raw):
+            if (w0_raw[raw_i] == 0x10
+                    and raw_i + 1 < len(w0_raw)
+                    and w0_raw[raw_i + 1] in {0x02, 0x03, 0x04}):
+                raw_i += 2
+            else:
+                raw_i += 1
+            stripped_so_far += 1
+        probe_skip = 7 + raw_i
+    else:
+        strt = b"STRT" + b"\xff\xfe" + bytes(14) + b"\x00"
+        probe_skip = 7 + 21
+
+    if len(strt) != 21:
+        return (b"", b"", b"")
+
+    # Separate terminator from data frames.
+    term_idx: Optional[int] = None
+    if a5_frames and a5_frames[-1].page_key != 0x0010:
+        term_idx = len(a5_frames) - 1
+
+    if term_idx is not None:
+        body_frames = a5_frames[:term_idx]
+        term_frame = a5_frames[term_idx]
+    else:
+        body_frames = a5_frames
+        term_frame = None
+
+    all_bytes = bytearray()
+    for fi, frame in enumerate(body_frames):
+        if fi == 0:
+            skip = probe_skip
+        elif fi in (1, 2):
+            skip = 13   # metadata pages
+        else:
+            skip = 12   # sample chunks
+        all_bytes.extend(_frame_body_bytes(frame, skip))
+
+    if term_frame is not None:
+        all_bytes.extend(_frame_body_bytes(term_frame, 11))
+
+    # Find the first valid `0e 08` footer marker.
+    footer_pos = -1
+    pos = 0
+    while True:
+        pos = bytes(all_bytes).find(b"\x0e\x08", pos)
+        if pos < 0 or pos + 26 > len(all_bytes):
+            break
+        yr = (all_bytes[pos + 4] << 8) | all_bytes[pos + 5]
+        if 2015 <= yr <= 2050:
+            footer_pos = pos
+            break
+        pos += 1
+
+    if footer_pos >= 0:
+        body = bytes(all_bytes[:footer_pos])
+        footer = bytes(all_bytes[footer_pos : footer_pos + 26])
+    elif len(all_bytes) >= 26:
+        body = bytes(all_bytes[:-26])
+        footer = bytes(all_bytes[-26:])
+    else:
+        body = bytes(all_bytes)
+        footer = b""
+
+    return (strt, body, footer)
+
+
 def write_blastware_file(
     event: Event,
     a5_frames: list[S3Frame],