mailbox: Phase 1 — typed-address tokens (RFC 014)

Add the value-token layer for typed addressing, ahead of the handle table:

- Addressable { type Msg }: minimal actor-type -> message-type hook (decision
  (a)). Raw actors are closures over channels; GenServer is multi-message and
  stays addressed via ServerRef. This gives the single-message actors a key.
- Addr<A>: direct, identity-bound address (RFC's Pid<A>, renamed to avoid the
  collision with the non-generic Pid identity). Plain Pid + PhantomData<fn()->A>.
- Name<A>: durable, re-resolving address; const-constructible (Name::new).

Both tokens are Copy + Send + Sync regardless of key (phantom is fn()->T), with
hand-written trait impls so no spurious T: Copy/Eq bounds leak in. No behavior
yet; send/resolve/storage land in later phases. Addr::new is #[cfg(test)] until
Phase 2 has a real (non-test) caller. Unit tests cover identity/copy/eq/debug
and Send+Sync.

src/lib.rs

@@ -54,7 +54,7 @@ pub use gen_server::{CallError, CallTimeoutError, CastError, GenServer, ServerBu
 pub use link::{link, trap_exit, unlink, ExitSignal};
 pub use monitor::{demonitor, monitor, Down, DownReason, Monitor, MonitorId};
 pub use mutex::{LockTimeout, Mutex, MutexGuard};
-pub use pid::Pid;
+pub use pid::{Addr, Addressable, Name, Pid};
 pub use pg::{join, leave, members, pick, Incarnation, Member, NodeId};
 pub use registry::{name_of, register, unregister, whereis, RegisterError};
 pub use runtime::{init, Config, Runtime};