Function collect_codec_closure 

pub fn collect_codec_closure(
    roots: &[TypeRef],
    types: &HashMap<String, TypeDecl>,
) -> (Vec<String>, Vec<GenericInst>)

v0.22b: the codec closure for a set of Json.encode/Json.decode[T] target type-refs — the named types needing per-type helpers (transitively through record fields and sum payloads) plus the generic instantiations needing specialised helpers. The same closure logic as the boundary collectors, rooted at expressions instead of service signatures.