Function phase_validate_providers 

pub fn phase_validate_providers(
    unit_tables: &HashMap<String, UnitTable>,
    groups: &BTreeMap<String, Vec<usize>>,
    parsed: &[ParsedFile],
    errors: &mut ErrorSink,
    tys: &Arc<Types>,
)

Phase 6c: validate that every (non-external) provider matches its capability exactly — each capability op has a provider op, and every provider op has a matching capability op with the same parameter and return types. Diagnostics go into errors.