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/find_blocks.py

@@ -0,0 +1,41 @@
+"""Search for structural repetition in the body bytes."""
+import sys
+sys.path.insert(0, ".")
+from analysis.load_bundle import load_bundle
+
+
+def find_pattern_offsets(body: bytes, pattern: bytes, max_count=20):
+    out = []
+    i = 0
+    while True:
+        i = body.find(pattern, i)
+        if i < 0:
+            break
+        out.append(i)
+        i += 1
+        if len(out) >= max_count:
+            break
+    return out
+
+
+def main():
+    for name in ("event-a", "event-b", "event-c", "event-d"):
+        b = load_bundle(name)
+        body = b.body
+        print(f"\n=== {name} (body={len(body)}, N_samples={len(b.samples['Tran'])}) ===")
+
+        # Try to find repeating substructures (look for 4-byte 0x10-prefixed markers)
+        for prefix in [b"\x10\x10", b"\x10\x04", b"\x10\x08", b"\x10\x0c", b"\x10\x18",
+                       b"\x10\x14", b"\x10\x20", b"\x10\x40", b"\x10\x80", b"\x10\x00",
+                       b"\x10\x01", b"\x10\x03", b"\x10\xf0", b"\xf1\x10", b"\x00\x10",
+                       b"\x40\x02", b"\x20\x04", b"\x30\x04", b"\x30\x08", b"\x00\x1a"]:
+            offs = find_pattern_offsets(body, prefix, max_count=200)
+            if 1 <= len(offs) <= 1000:
+                # Print first 10 offsets
+                first = offs[:6]
+                last = offs[-3:]
+                print(f"  '{prefix.hex()}' x{len(offs):>4}  first={first} last={last}")
+
+
+if __name__ == "__main__":
+    main()