StageInput

Where a stage's set comes from, and WHAT IT IS FOR.

This is the type that closes the gap. Before it, an invocation could carry only a literal
bounds: Option<IdSet> and had no way to name a producing stage.

[StageRelation] is a field of every variant rather than an Option on the invocation, so
"an input with no declared relation" is unrepresentable rather than merely invalid. The
relation belongs to the edge, not to the stage.

{
    "as": "bound",
    "from": "caller",
    "ids": {
        "ids": [
            "497f6eca-6276-4993-bfeb-53cbbbba6f08"
        ],
        "kind": "resource",
        "provenance": null
    }
}