Support list-valued @nodeId+@reference on INSERT inputs (row fan-out)

On the read side, [ID!] @nodeId filter leaves are a designed shape (IN predicate, see NodeIdReferenceFilterPipelineTest), but on the write side a list-valued node-id reference on an INSERT input has no semantics: the input maps to one inserted row with one FK slot, so a field like parentIds: [ID!]! @nodeId(typeName: "T") @reference(path: […​]) would have to mean row fan-out, one inserted row per decoded id, with the sibling scalar fields repeated across the fanned-out rows. Motivating case: FS’s OpprettUtdanningsspesifikasjonshierarkiInput, where callers today must repeat the whole input element per parent id. Deciding whether fan-out is wanted at all is part of this item; if yes, the INSERT emitter’s per-cell binding (TypeFetcherGenerator.buildInsertDecodeLocals / buildPerCellValueList) needs a list-aware decode-and-expand path, and the R419 build-time rejection gets lifted for the supported shape. Related: R57 covers list arity for the TranslatedFk join-translation case on the read side; this item is the DirectFk write path.