fix(codec): 40 NN int16 blocks are not capped at NN=8

data_block_len() rejected any `40 NN` block with NN > 0x08. That guard had
no evidence behind it: every corpus available when it was written used only
NN in {1,2,3,4,8}, so it was never exercised. Loud UM12947 events use NN of
12, 16, 20 ... up to 196.

Because walk_body/run stop at the first unrecognised tag rather than
raising, rejecting those blocks surfaced as silently short channels -- e.g.
Tran 1812 / Vert 2132 / Long 2324 on a file whose export carries 2324 for
all three. The real bound is the buffer; the caller additionally clamps to
the record end.

Verified against Thor's own CSV exports for UM12947 (2025-07-14 .. 09-25,
167 waveforms, supplied as CSV.zip):

  length mismatches   22 -> 0
  per-sample exact    1,476,242 / 1,476,249

These are NOT truncated recordings, which was the competing hypothesis --
the exports carry the full sample count.

tests/test_waveform_codec.py asserted the cap as intended behaviour. That
assertion encoded an assumption, not a verified fact, and is replaced with
one pinning the opposite plus the evidence.

Across all three ground-truth corpora: 459 waveform files,
3,807,158 / 3,807,165 samples exact. Production IDFW is now 575/575 with
zero truncations and zero decode failures (median PPV error -0.0007% across
8 units). Series-3 re-verified unchanged at 14,338/14,338.

The 7 residual samples each differ by one 4th-decimal tick and are Thor's
own rounding: intersecting the per-sample rounding constraints over that
corpus is infeasible (binding pair contradict by 2.3e-11, 7e-5 relative),
so no single linear LSB reproduces every printed value. _GEO_LSB_IPS is
already pinned to ~1e-11; do not retune it to chase these.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ru8Lg9HkkYvX9VWWo65SmL

minimateplus/waveform_codec.py

@@ -758,7 +758,18 @@ def data_block_len(body: bytes, p: int) -> Tuple[Optional[int], Optional[int]]:
     hi = t0 & 0xF0
     nn = ((t0 & 0x0F) << 8) | t1
     if hi == 0x40:                          # int16 BE data block
-        return (None, None) if (nn == 0 or nn > 0x08) else (2 * nn + 2, nn)
+        # NN was capped at 0x08 until 2026-09-11.  That cap had no basis: the
+        # two corpora available at the time only ever used NN in {1,2,3,4,8},
+        # so it was never exercised.  Loud UM12947 events use NN of 12, 16,
+        # 20 ... up to 196, and every value above 8 halted the walk, which
+        # surfaced as silently short channels (walk_body/run stop at the first
+        # unrecognised tag rather than raising).  Verified against Thor's own
+        # exports: 22 length-mismatched files -> 0, and the affected corpus
+        # went to 1,476,242/1,476,249 samples exact.  The real bound is the
+        # buffer; the caller additionally clamps to the record end.
+        if nn == 0 or p + 2 * nn + 2 > len(body):
+            return None, None
+        return 2 * nn + 2, nn
     if nn == 0 or nn % 4:
         return None, None
     if hi == 0x00: