D1313 — A nested frontmatter value is refused, never written as text

Decision. OKF frontmatter stays the flat YAML subset. A value that is a map, or a list holding a map or a list, is refused by every write tool (concept_write, concept_batch, concept_patch) with invalid_frontmatter, naming the key and saying nested values are not supported (flatten the keys or put the structure in the body). serializeEntry returns an error for a type it does not know; it has no %v default branch.

Why. A map printed with %v became the string map[a:1]: the write answered findings: [] and the agent reported nested data as stored. Silent data corruption is worse than a refusal, and the refusal tells the agent what to do.

Alternatives rejected. Serializing nested values as YAML: the parser, lint and the round-trip check are built on the flat subset (D291 keeps only opaque blocks), so it would widen the format for every reader. Coercing to JSON text: a string that looks structured is the same lie in a different form.

Consequences. applyFrontmatterMap validates before touching the frontmatter, and WriteConcept runs Frontmatter.Validate as a second guard. A future new frontmatter type must be added to serializeEntry explicitly.