mailbox: Phase 4a — direct identity-bound send to Pid<A> (RFC 014 §4.2)

Make a Pid<A> messageable: install + send_to, the direct addressing mode that
dies with the actor and never redirects (the counterpart to the re-resolving
Name).

- install::<A: Addressable>(tx: Sender<A::Msg>) -> Pid<A>: opt-in, lazy,
  nameless publish (RFC 014 §5). Files the actor's inbox into the by_index
  mailbox table under TypeId::of::<A::Msg>() and re-types self's identity as
  Pid<A>. Infallible: no name to collide on, self is always live in run().
- register and install now share publish_channel (the insert-or-extend-mailbox
  step factored out); register additionally binds the name. Byte-identical
  storage, so name- and pid-addressing populate the same table.
- send_to::<A>(Pid<A>, A::Msg): resolve by raw pid, deliver with NO redirect.
  The stored mailbox must be this exact incarnation (generation included) and
  live, else SendError::Dead — even when the slot now holds a different live
  actor, which is left untouched. Resolution clones the Sender under the Leaf
  lock, releases, then sends (Leaf -> Channel), as name send / pg / finalize.
- send_to_pid is the shared raw-pid core; send_dyn (§4.6) lands on it next.
- SendError gains Dead(M), the pid-path counterpart to name-path Unresolved;
  into_inner / Display / Debug updated. Name send never yields Dead, pid send
  never yields Unresolved — disjoint by construction, documented on the enum.

tests/registry.rs: install_then_send_to_pid_delivers; and the load-bearing
send_to_does_not_redirect_after_takeover (slot reused by a new incarnation, the
stale Pid<A> send returns Dead and the new occupant gets only its own message).
Full suite + order-checker green, warning-free across all targets.

src/lib.rs

@@ -56,7 +56,7 @@ pub use monitor::{demonitor, monitor, Down, DownReason, Monitor, MonitorId};
 pub use mutex::{LockTimeout, Mutex, MutexGuard};
 pub use pid::{Addressable, Erased, Name, Pid, RawPid};
 pub use pg::{join, leave, members, pick, Incarnation, Member, NodeId};
-pub use registry::{register, send, unregister, whereis, RegisterError, SendError};
+pub use registry::{install, register, send, send_to, unregister, whereis, RegisterError, SendError};
 pub use runtime::{init, Config, Runtime};
 pub use scheduler::{
     block_on_io, request_stop, run, self_pid, sleep, spawn, spawn_under, wait_readable,