Reject list-valued @nodeId+@reference carriers on INSERT inputs at build time

A list-valued node-id reference field on an INSERT input (e.g. parentId: [ID!]! @nodeId(typeName: "T") @reference(path: […​])) passes classification and validation today: NodeIdLeafResolver is arity-agnostic, BuildContext.classifyInputField just bakes list=true into the ColumnReferenceField / CompositeColumnReferenceField carrier, and MutationInputResolver.admitMutationInputFields admits reference carriers for INSERT unconditionally (only the NestingField arm rejects lists). The generated code compiles but hardcodes single-value assumptions (instanceof String decode guard, .value1() bind in TypeFetcherGenerator), so any non-empty list value throws GraphitronClientException "Decoded NodeId did not match the expected type" at runtime. That is the worst failure mode: it surfaces only when the field is populated. Until fan-out semantics are actually supported (R420), a list() guard in admitMutationInputFields alongside the existing nesting-field list rejection should turn this into a clear build-time schema error.