codec-re: crack Tran channel codec with high-amplitude May 11 bundle

User uploaded 3 high-amplitude events (PPV 6-7 in/s — shook the geophone
hard) to decode-re/5-11-26/.  These cracked the Tran codec:

- Preamble bytes [3:5] and [5:7] = Tran[0] and Tran[1] as int16 BE
  in 16-count units (LSB = 0.005 in/s).  Confirmed across all 7
  fixtures.
- First data block carries Tran deltas from sample 2 onward:
  * 10 NN block: NN/2 bytes of payload, each byte = two 4-bit signed
    nibble deltas (high nibble first)
  * 20 NN block: NN int8 signed deltas

Verified 22+42+46 = 110 Tran samples across SP0/SS0/SV0 with 0 errors
against BW's ASCII export.

Why the earlier 96-combination brute force failed: the quiet 5-8
events all had T[0] = T[1] ≈ 0 so the preamble's per-channel encoding
was undetectable.  Loud events made the encoding obvious.

What's solved:
- minimateplus.waveform_codec.decode_tran_initial: returns first
  N Tran samples in 16-count units for any body.
- Walker length formula for in-data 30 NN blocks (NN*2 instead of NN*4).
- Walker now handles bodies that start with 20 NN (in addition to 10 NN).

What's still open:
- Tran past the first data block (multi-block channel switching).
- Vert / Long / MicL channel encodings.
- Walker correctness past offset ~427 in event-b.

Tests: 36 pass.  decode_waveform_v2 still returns None — the full
multi-channel decoder is not wired up.  decode_tran_initial is the
new verified entry point.

Files: minimateplus/waveform_codec.py, tests/test_waveform_codec.py
(adds 5-11-26 fixtures + decode_tran_initial tests), and
docs/instantel_protocol_reference.md §7.6.1 (Tran codec spec).

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