Where a stage's input set came from: an earlier stage, or the caller.[renamed from BoundsSource — 2026-08-08] It reports where an input came from, which is
direction-neutral, and the surrounding fields no longer say "bounds" — an input may be a bound
or a seed, and half the compositions this surface exists for alternate between them.
{
"source": "upstream",
"stage": "string"
}