codec-re: solve waveform body block framing; per-byte sample mapping still open

Decoded the structural framing of the Blastware waveform body — the bytes
between the 21-byte STRT record and the 26-byte file footer.  The body is
a sequence of tagged variable-length blocks, NOT raw int16 LE.  Five tag
types (10/20/00/30/40 NN) and their lengths are now confirmed against the
4-event May 2026 fixture bundle.  Body splits cleanly into ~16 segments
(for a 1280-sample event) separated by 40 02 segment headers carrying a
monotonically incrementing uint32 LE counter at bytes [8:12].

What's done:
- minimateplus/waveform_codec.py — block walker, segment splitter, segment
  header parser.  decode_waveform_v2 is a stub returning None until the
  byte-to-sample mapping is solved; client.py is unchanged.
- tests/test_waveform_codec.py — 31 tests covering block detection, lengths,
  contiguous-walk, segment splitting, segment-header parsing, and counter
  monotonicity.  All pass.
- tests/fixtures/decode-re-5-8-26/ — bundled fixtures (4 events, BW binary
  + Blastware ASCII export each).
- docs/instantel_protocol_reference.md §7.6.1 — replaced retraction box
  with the verified structural decoding plus an explicit list of what's
  still open.

What's still open: the per-byte mapping inside 10 NN / 20 NN blocks.  96
channel-permutation × nibble-order × sign-convention combinations were
brute-force tested; none match BW's ASCII export to within ±1 ADC count.
The codec is more elaborate than uniform 4-bit deltas — likely a hybrid
variable-bit-width scheme with segment-anchor resync points.  Next
recommended step: capture an event with a known calibration tone to pin
down magnitude scaling.

Walker also bails out partway through event-b (open issue documented in
both the module and the protocol reference).

analysis/walk_v3.py

@@ -0,0 +1,51 @@
+"""Walk chunks; auto-detect preamble length by finding first 10 NN."""
+import sys
+sys.path.insert(0, ".")
+from analysis.load_bundle import load_bundle
+
+
+def walk_chunks(body, start, max_NN=0x80):
+    chunks = []
+    i = start
+    while i + 1 < len(body):
+        if body[i] != 0x10:
+            break
+        NN = body[i + 1]
+        if NN == 0 or NN > max_NN or NN % 4 != 0:
+            break
+        chunk_len = NN // 2 + 4
+        if i + chunk_len > len(body):
+            break
+        data = bytes(body[i + 2 : i + 2 + NN // 2])
+        trailer = bytes(body[i + 2 + NN // 2 : i + chunk_len])
+        chunks.append((i, NN, data, trailer))
+        i += chunk_len
+    return chunks, i
+
+
+def find_first_chunk_start(body):
+    """Locate first byte that begins a `10 NN` chunk (NN ∈ multiples of 4, 4..0x7C)."""
+    for i in range(20):
+        if body[i] == 0x10 and body[i + 1] % 4 == 0 and 0 < body[i + 1] <= 0x7C:
+            return i
+    return -1
+
+
+def main():
+    for name in ("event-c", "event-d", "event-a", "event-b"):
+        b = load_bundle(name)
+        body = b.body
+        start = find_first_chunk_start(body)
+        chunks, end = walk_chunks(body, start)
+        print(f"\n=== {name} ===  body={len(body)}  N_samples={len(b.samples['Tran'])}  start={start}")
+        print(f"  chunks parsed: {len(chunks)}, walk ended at {end}")
+        if chunks:
+            print(f"  first 5 chunks: {[(c[0], c[1], c[3].hex()) for c in chunks[:5]]}")
+            print(f"  last 5 chunks: {[(c[0], c[1], c[3].hex()) for c in chunks[-5:]]}")
+            print(f"  bytes around end of walk: {body[end-4:end+12].hex(' ')}")
+        else:
+            print(f"  bytes at start: {body[start:start+16].hex(' ')}")
+
+
+if __name__ == "__main__":
+    main()