Project a key column across a list of node ids into an array-valued parameter

An argMapping path may open a @nodeId-carrying ID with one of its node type’s key columns. Opening a [ID!] carrying the same directive is rejected, and the rejection is a scope limit rather than a rule: a list of node ids of one type opens into the list of that key column across the decoded ids, which is a perfectly determinate value. The decode side already has the shape, decode<Record>List materialising one record per element and CompositeDecodeHelperRegistry returning List<T> on the list axis, so what is missing is the consuming end.

Two things have to be decided rather than just built. A routine IN parameter taking the projection would have to be array-typed, so the binding’s type gate has to compare the column’s Java type against an array parameter rather than a scalar one, and a mismatch there wants the same message the scalar gate owes (filed separately). And the emitted read has to say what an absent or malformed element means: the scalar form decodes to null and lets the parameter carry it, while a list has the further choice between a null element, a shorter list, and a request-time error. Pick one and state it, rather than letting the emitter’s shape decide by accident.

Until then the walk rejects the shape with a message that says it does not emit yet, which is honest but is a rejection standing where a deferral belongs: ArgBindingMap.of’s `Result has no deferral arm, and adding one would be new vocabulary on the walk surface that the strangler migration is draining. Worth revisiting here whether the routine resolver’s own Rejection.deferred should carry it instead.