The frame-chunk path still encoded compressed block headers directly in C even
though the equivalent single-block path had moved to the Rust serializer.
Route this remaining call site through the same ABI leaf so both paths retain
one byte-formatting implementation and identical RLE/compressed handling.
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
c9490ffbcd