Handle a scope-based selector node (created via addSelectorFunction). The stage function IS the selector — its return value contains branch IDs. Execution order: runStage(fn) → commit → resolve children → parallel execute.
Optional
Handle a scope-based selector node (created via addSelectorFunction). The stage function IS the selector — its return value contains branch IDs. Execution order: runStage(fn) → commit → resolve children → parallel execute.