HostTypeRequirement::kCollectionlessSourceRunOnceAnyNode (formerly kRunOnceAnyNode) has a semantic invariant that should be enforced programmatically: stages using this host type are supposed to provide their own data and be independent of any collection, which means requiresInputDocSource == false and isIndependentOfAnyCollection == true.

There is one exception: the $lookup subpipeline code path hits kCollectionlessSourceRunOnceAnyNode with requiresInputDocSource == true. We should add a tassert in StageConstraints (in the constructor or validation block) that always fires when hostRequirement == kCollectionlessSourceRunOnceAnyNode and confirms that (!requiresInputDocSource && isIndependentOfAnyCollection) or we're in a $lookup subpipeline.