RFC 016 Chunk 3: parentage tree view

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.

src/lib.rs

@@ -56,7 +56,8 @@ pub use gen_server::{
     NamedServerBuilder, ServerBuilder, ServerCtx, ServerName, ServerRef, TimerHandle, Watcher,
 };
 pub use introspect::{
-    actor_info, snapshot, ActorInfo, ActorState, RuntimeSnapshot, SNAPSHOT_FORMAT_VERSION,
+    actor_info, snapshot, tree, tree_from, ActorInfo, ActorState, RuntimeSnapshot, RuntimeTree,
+    TreeNode, SNAPSHOT_FORMAT_VERSION,
 };
 pub use link::{link, trap_exit, unlink, ExitSignal};
 pub use monitor::{demonitor, monitor, Down, DownReason, Monitor, MonitorId};