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/runtime.rs

@@ -457,6 +457,14 @@ impl Slot {
         self.word.generation()
     }
 
+    /// Raw packed state word, for introspection's lock-free classify
+    /// (`introspect.rs`). The coarse `status_for` only distinguishes
+    /// Live/Done/Stale; the snapshot needs the fine scheduling state.
+    #[inline]
+    pub(crate) fn state_word(&self) -> u64 {
+        self.word.load()
+    }
+
     /// A pid's-eye snapshot of the slot. Cold paths re-read this under the
     /// cold lock (generation can't change while it is held).
     #[inline]