The public sequence compression paths still built compressed block headers
locally in C, leaving two byte-formatting implementations after the frame-chunk
path was migrated. Route both compressed-block branches through the existing
Rust serializer while retaining their C-side sequence, entropy, and capacity
logic.
Test Plan:
- `make -B -C lib -j2 lib` -- passed
- `make -B -C tests -j2 fuzzer` -- passed
- `tests/fuzzer -s4560 -t47 -i48 -v` -- passed
- `git diff --cached --check` -- passed
ddidderr
ac7683de9f