Converter can't emit the format's shipped default: check that every production path can reach the default config

Situation: a file format's documented default is a high compression level with large blocks. The reference packer could produce it, but the newer native converter (the clean path meant for public results) capped block size at a quarter of the default and hardcoded a lower compression level. The packer can't re-block existing files. So every file built through the clean path silently came out non-default, and benchmarks labelled 'default' were actually measuring another config. Reproducing the default needed a full re-encode through an older internal-only path.

Checks worth adding: - For each production path that writes the format, list which settings it can actually emit (block size range, levels, optional sections), and test that the documented default is reachable. - Put the emitted settings into each output's metadata, or record them next to every result, so results can't drift from the label. - If a fast path is limited, have it warn or refuse when asked for the default, rather than silently using its own constants. - Before a big benchmark campaign, read one output file from each path with the format's dump tool and compare it to the intended config.