mailbox: Phase 4b — send_dyn bare-pid escape hatch (RFC 014 §4.6)

The explicit fallible fallback for when only an untyped Pid is in hand (off a
Down, out of a future members()), so the typed send_to / send-by-name stay the
default and callers don't grow workarounds around a missing primitive.

- send_dyn::<M>(Pid, msg): one line over the shared send_to_pid core. Same
  identity-bound, no-redirect liveness as send_to (Dead once the incarnation is
  gone); the difference is the explicit M, so NoChannel is a real runtime
  outcome here rather than the debug_assert it collapses to on the typed paths.
- Takes Pid<Erased> specifically: a typed Pid<A> must .erase() to reach it, so
  opting into the fallible downcast stays visible at the call site.

tests/registry.rs: send_dyn_delivers_and_reports_wrong_type (right type
delivers; live actor with no channel for the asked type -> NoChannel) and
send_dyn_to_dead_pid_is_dead. Full suite + order-checker green, warning-free.

This completes the RFC 014 send surface: Name<M> (re-resolving), Pid<A>
(typed direct), send_dyn (bare-pid). send_after (§6) can now target any of
them — its Dest question is answered.

src/lib.rs

@@ -56,7 +56,9 @@ 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::{install, register, send, send_to, unregister, whereis, RegisterError, SendError};
+pub use registry::{
+    install, register, send, send_dyn, 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,