tree() / tree_from() fold a Chunk-1 snapshot into a forest by grouping
each actor under its parent pid — one O(n) pass, no new reads. tree_from
is public so a held (or synthetic) snapshot can be folded without a
second scan.
- D8: actors parented at ROOT_PID are genuine roots; an actor whose
recorded parent is absent from the snapshot is re-rooted under the
sentinel and flagged orphaned, so the forest stays total. take()-on-
place doubles as a guard against re-entering a node.
- D9: the edge is parent/spawned-by, documented as not necessarily
supervision.