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_chunks.py

@@ -0,0 +1,44 @@
+"""Walk the body assuming chunks delimited by 0x10 NN tags. Print each chunk's structure."""
+import sys
+sys.path.insert(0, ".")
+from analysis.load_bundle import load_bundle
+
+
+def walk(body: bytes, start_offset: int = 7, max_chunks: int = 30):
+    """Find all positions where byte = 0x10 followed by a multiple-of-4 byte. Print chunks."""
+    chunks = []
+    i = start_offset
+    while i < len(body) - 1:
+        # Find next `10 NN` where NN is multiple of 4 (and not preceded by another 0x10 immediately, which would be data).
+        if body[i] == 0x10 and (body[i+1] % 4 == 0):
+            chunks.append(i)
+        i += 1
+    return chunks
+
+
+def main():
+    for name in ("event-c", "event-d"):
+        b = load_bundle(name)
+        body = b.body
+        positions = []
+        i = 7  # skip 7-byte preamble
+        while i < len(body) - 1:
+            if body[i] == 0x10 and body[i+1] % 4 == 0 and body[i+1] > 0:
+                positions.append(i)
+                i += 2  # skip past tag
+            else:
+                i += 1
+        print(f"\n=== {name} ===  body={len(body)}, total `10 NN` (NN%4==0, NN>0) tags: {len(positions)}")
+        # Print first 20 chunks: show position, NN, gap to next tag
+        for k in range(min(30, len(positions))):
+            pos = positions[k]
+            NN = body[pos + 1]
+            next_pos = positions[k+1] if k+1 < len(positions) else len(body)
+            gap = next_pos - pos
+            data_bytes = body[pos+2 : next_pos]
+            print(f"  chunk[{k:>3}] @ {pos:>5}  NN=0x{NN:02x} ({NN:>3}, NN/2={NN//2})  gap={gap:>3}  "
+                  f"data={data_bytes[:24].hex(' ')}{'...' if len(data_bytes) > 24 else ''}")
+
+
+if __name__ == "__main__":
+    main()