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/brute_force.py

@@ -0,0 +1,93 @@
+"""Brute-force test channel permutations / nibble orders on event-d (simplest signal)."""
+import sys
+import itertools
+sys.path.insert(0, ".")
+from analysis.load_bundle import load_bundle
+from minimateplus.waveform_codec import walk_body
+
+
+def s4(n):
+    return n if n < 8 else n - 16
+
+
+def decode(body, channel_perm, nibble_order, sign_mode, init_from_header):
+    """Try one decoder configuration on event-d. Returns first 8 cumulative samples per channel."""
+    blocks = walk_body(body)
+    # Initial values from bytes [4:7] if init_from_header else 0
+    if init_from_header:
+        init = [body[4] if body[4] < 128 else body[4] - 256,
+                body[5] if body[5] < 128 else body[5] - 256,
+                body[6] if body[6] < 128 else body[6] - 256,
+                0]
+    else:
+        init = [0, 0, 0, 0]
+    cur = list(init)
+    out = [[init[0]], [init[1]], [init[2]], [init[3]]]  # sample 0 = init
+    nibble_idx = 0  # within delta stream; channel = channel_perm[nibble_idx % 4]
+
+    # Walk only the 10 NN data blocks
+    for blk in blocks:
+        if blk.tag_hi != 0x10:
+            continue
+        for byte in blk.data:
+            if nibble_order == 'high_first':
+                nib1, nib2 = (byte >> 4) & 0xF, byte & 0xF
+            else:
+                nib1, nib2 = byte & 0xF, (byte >> 4) & 0xF
+            for nib in (nib1, nib2):
+                if sign_mode == 'signed':
+                    delta = s4(nib)
+                else:
+                    delta = nib
+                ch = channel_perm[nibble_idx % 4]
+                cur[ch] += delta
+                if (nibble_idx + 1) % 4 == 0:
+                    out[0].append(cur[0])
+                    out[1].append(cur[1])
+                    out[2].append(cur[2])
+                    out[3].append(cur[3])
+                nibble_idx += 1
+                if len(out[0]) >= 16:
+                    return out
+    return out
+
+
+def best_match(pred, truth, n=10):
+    """Sum of squared differences in first n samples."""
+    n = min(n, len(pred), len(truth))
+    return sum((pred[i] - truth[i])**2 for i in range(n))
+
+
+def main():
+    b = load_bundle("event-d")
+    # truth in 16-count units
+    tr = {ch: [round(v * 200) for v in b.samples[ch]] for ch in ("Tran", "Vert", "Long")}
+
+    print("Truth event-d first 10 samples:")
+    for ch in ("Tran", "Vert", "Long"):
+        print(f"  {ch}: {tr[ch][:10]}")
+
+    # Test 96 combinations
+    best = []
+    for perm in itertools.permutations([0, 1, 2, 3]):
+        for nibble_order in ('high_first', 'low_first'):
+            for sign in ('signed', 'unsigned'):
+                for init_h in (False, True):
+                    decoded = decode(b.body, perm, nibble_order, sign, init_h)
+                    # Score as TVL channel-sum
+                    score = sum(
+                        best_match(decoded[i], tr[ch], n=10)
+                        for i, ch in enumerate(("Tran", "Vert", "Long"))
+                        if i < 3
+                    )
+                    label = f"perm={perm} nib={nibble_order[:1]} sign={sign[:3]} init={init_h}"
+                    best.append((score, label, decoded))
+
+    best.sort(key=lambda x: x[0])
+    print(f"\nTop 10 configurations:")
+    for s, lbl, dec in best[:10]:
+        print(f"  score={s:>5}  {lbl}  T={dec[0][:8]}  V={dec[1][:8]}  L={dec[2][:8]}")
+
+
+if __name__ == "__main__":
+    main()