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_v2.py

@@ -0,0 +1,50 @@
+"""Deterministic chunk walker: each chunk = [10 NN][NN/2 bytes data][2 bytes trailer]."""
+import sys
+sys.path.insert(0, ".")
+from analysis.load_bundle import load_bundle
+
+
+def walk_chunks(body: bytes, start: int = 7):
+    """Yield (offset, NN, data_bytes, trailer_bytes) tuples."""
+    i = start
+    while i + 1 < len(body):
+        if body[i] != 0x10:
+            break
+        NN = body[i + 1]
+        if NN == 0 or NN > 0x80 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])
+        yield (i, NN, data, trailer)
+        i += chunk_len
+
+
+def main():
+    for name in ("event-c", "event-d", "event-a", "event-b"):
+        b = load_bundle(name)
+        body = b.body
+        chunks = list(walk_chunks(body))
+        print(f"\n=== {name} ===  body={len(body)}  N_samples={len(b.samples['Tran'])}")
+        print(f"  chunks parsed: {len(chunks)}")
+        if chunks:
+            last = chunks[-1]
+            end_of_walk = last[0] + last[1] // 2 + 4
+            print(f"  walk ended at offset {end_of_walk} (= {len(body) - end_of_walk} bytes from end)")
+            # Stats
+            total_data_bytes = sum(len(c[2]) for c in chunks)
+            print(f"  total data bytes: {total_data_bytes}, total nibbles: {2*total_data_bytes}")
+            if name in ("event-c", "event-d"):
+                ratio = (2 * total_data_bytes) / (len(b.samples['Tran']) * 4)
+                print(f"  nibbles per (sample × channel): {ratio:.3f}")
+            # Sum of trailer second-byte
+            trailer_sums = [c[3][-1] if c[3] else None for c in chunks]
+            print(f"  first 10 chunks: {[(c[0], c[1], c[3].hex()) for c in chunks[:10]]}")
+            # Print last 10 chunks (likely transition to trailer)
+            print(f"  last 10 chunks: {[(c[0], c[1], c[3].hex()) for c in chunks[-10:]]}")
+
+
+if __name__ == "__main__":
+    main()