Function validate_section_keys 

pub fn validate_section_keys<'a>(
    provided: impl Iterator<Item = &'a str>,
    schema: &TypeDefinition,
) -> Result<(), ValidationError>

Validate that every section key in provided is either schema-declared for schema, or — if the schema has a catch-all section — admitted by it. Unknown keys return ValidationError::UnknownSection carrying the declared list plus a Levenshtein suggestion (or the catch-all key when no close match exists).

Pure function: no I/O, no allocation outside the eventual error payload. The "relationships" section is allowed through here — the engine layer above gates it via its own SectionNotUpdatable check.