RFC 016 Chunk 1: runtime introspection read primitive

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.

src/lib.rs

@@ -27,6 +27,7 @@ pub mod registry;
 pub mod pg;
 pub mod link;
 pub mod gen_server;
+pub mod introspect;
 pub mod runtime;
 pub(crate) mod raw_mutex;
 pub(crate) mod slot_state;
@@ -54,6 +55,9 @@ pub use gen_server::{
     call, cast, shutdown, whereis_server, CallError, CallTimeoutError, CastError, GenServer,
     NamedServerBuilder, ServerBuilder, ServerCtx, ServerName, ServerRef, TimerHandle, Watcher,
 };
+pub use introspect::{
+    actor_info, snapshot, ActorInfo, ActorState, RuntimeSnapshot, SNAPSHOT_FORMAT_VERSION,
+};
 pub use link::{link, trap_exit, unlink, ExitSignal};
 pub use monitor::{demonitor, monitor, Down, DownReason, Monitor, MonitorId};
 pub use mutex::{LockTimeout, Mutex, MutexGuard};