v2.65.5 — Sprint 135: reference-decoder default-format guard in make test

Test-infrastructure only; the shipping codec, wire format, and all
outputs are unchanged.

Sprint 134 fixed the reference decoders' missing HUFFMAN4 support — a
gap that had let the differential cross-check silently skip default
encoder output. This release makes that class of gap impossible to
reintroduce unnoticed: a new guard, tests/reference_roundtrip.py,
now runs in both make test and make python-test. It

• builds deterministic literal-heavy fixtures and compresses them in
  balanced and extreme mode,
• walks the .vv container and FAILS if no 'S'/'T' block with
  lit_fmt = 4 was produced (so the guard cannot pass vacuously if
  encoder format selection changes),
• decodes every stream with the Python reference decoder and compares
  byte-for-byte, and
• when node is available, repeats the byte-exact check with the
  JavaScript reference decoder via tests/reference_decode_check.js.

The guard's failure path is verified both ways: the pre-Sprint-134
Python reference fails it with NotImplementedError, and the JS checker
exits non-zero on an intentional plaintext mismatch.

Validation: guard passes 8/8 checks with 4 HUFFMAN4 blocks exercised;
22/22 C suites; 5,200 differential fuzz cases; JS reference suite
17/17; ratio gate ±0.