Writer×reader matrix finds serializer bugs single-library fuzzing misses

For format libraries (YAML here), dump a fixed corpus of awkward strings (type-lookalikes like NO/1_000/1e3, leading/trailing whitespace, U+0085/U+2028, C1 controls, BOM, >1024-char keys, multi-line with odd leading whitespace) in several positions (value, key, sequence item, nested, document root) with every serializer, then load each output with every parser. The expected value is the input, so no majority vote is needed. Then split failing cells by cause before reading individual cases: schema split, line-break chars, non-printables, root block-scalar indentation, other. Most cells are legitimate spec disagreements, and the real bugs sit in the 'other' bucket or in rows where even the writer's own reader fails. Across 17 writers × 20 readers this found new bugs in ruamel.yaml, sigs.k8s.io/yaml and KYAML in about a day, all of which had passed their own round-trip or fuzz tests. Check each finding against the upstream tracker before patching, because a lot of it is already filed.