§5 records the post-OAI-12 from_yaml contract: duplicate keys rejected
loudly on YAML (with the empirical correction that serde_json 1.0.151's
Value path last-wins rather than errors — the YAML side is the stricter
one), non-finite floats rejected with JSON-pointer context, merge keys
applied via apply_merge (shallow, referencing keys win; the one
deliberate YAML 1.2 deviation), scalar-key stringification matching the
core schema, and the no-new-bounds note (the walk stays inside
yaml_serde's parse-time limits).
Verification: cargo doc --no-deps clean; module-doc cross-check in
openapi_spec.rs matches this contract.