snapshot() / actor_info() return owned ActorInfo over the slab: pid,
names, fine-grained scheduling state, parent edge, trap flag, mailbox
depth, and monitor/link/joiner counts. Pure reads, no hot-path change.
- ActorState maps the packed slot word (no new storage); introspect.rs.
- D1: RuntimeSnapshot carries SNAPSHOT_FORMAT_VERSION from day one.
- D2: per-slot tearing (ps semantics); actor_info coherent per actor.
- D3: mailbox depth included. Registry channels now stored behind an
ErasedSender trait (downcast for clone_sender + type-erased
queued_len); depth summed over published channels under the registry
Leaf (Leaf -> Channel), kept out of the cold-lock pass so no two
Leaves are ever held at once. Depth covers published channels only.
- Done slots surface as root-less tombstones.
