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/channel.rs

@@ -137,6 +137,14 @@ impl<T> Drop for Receiver<T> {
 }
 
 impl<T> Sender<T> {
+    /// Number of messages currently queued behind this channel. Introspection
+    /// only (RFC 016 mailbox depth); takes the channel lock, so callers reach
+    /// it under the registry Leaf (Leaf → Channel) via the erased probe in
+    /// `registry.rs`, never on a hot path.
+    pub(crate) fn queued_len(&self) -> usize {
+        self.inner.lock().queue.len()
+    }
+
     pub fn send(&self, value: T) -> Result<(), SendError<T>> {
         let unpark = {
             let mut g = self.inner.lock();