mailbox: typed-path producers, by-name gen_servers, root-exit teardown (RFC 014)

Implements the new addressing surface the examples in examples/ specify:

- spawn_addr::<A>(FnOnce(Receiver<A::Msg>)) -> Pid<A>: the typed-path
  producer. Makes the inbox, spawns the body with its receiver, and publishes
  the sender from the PARENT side (registry::install_for) before returning the
  pid, so an immediate send_to always resolves (no race on the body installing
  itself). Detached, like ServerBuilder::start.

- lookup_as / pick_as / members_as: re-type an erased pid as Pid<A> over the
  heterogeneous registry/pg stores, sharing one helper (pid::assert_type).
  Unchecked but sound: routing is by message TypeId, so a wrong A degrades to
  SendError::NoChannel on the next send, never a misdelivery.

- dispatch::<A>(group, msg) -> Result<Pid<A>, SendError<A::Msg>>: pick_as +
  send_to, with the message handed back on failure. New SendError::NoMember
  variant for the empty/all-dead group case.

- gen_server naming: ServerName<G> backed internally by the registry's existing
  typed-channel store keyed by TypeId::of::<Envelope<G>>() — Envelope stays
  private, no separate directory. Type-state NamedServerBuilder<G> keeps the
  current infallible ServerBuilder::start() untouched; its start() is fallible
  (parent-side register, NameTaken). Free call/cast/whereis_server resolve per
  use. ServerRef::shutdown (+ free shutdown) is the sys-style synchronous stop.

- root-exit teardown: the run's initial actor is recorded as root; when it
  finalizes it flags root_exited, and the scheduler's idle verdict then stops
  the parked-forever remainder (a one-shot RootDrain sweep). Deferred to the
  idle point on purpose: the run queue drains first, so actors with queued work
  finish rather than unwinding on the stop. This lets a named-server daemon (or
  any pinned actor) wind the run down instead of hanging on live_actors > 0.

request_stop is refactored to a request_stop_inner core so the sweep can drive
it from inside the runtime without re-borrowing the thread-local.

Lib + tests + examples build warning-free; full suite green.

src/lib.rs

@@ -50,18 +50,22 @@ pub use channel::{
     channel, select, select_timeout, try_select, try_select_timeout, Receiver, RecvError,
     RecvTimeoutError, Selectable, Sender,
 };
-pub use gen_server::{CallError, CallTimeoutError, CastError, GenServer, ServerBuilder, ServerCtx, ServerRef, Watcher};
+pub use gen_server::{
+    call, cast, shutdown, whereis_server, CallError, CallTimeoutError, CastError, GenServer,
+    NamedServerBuilder, ServerBuilder, ServerCtx, ServerName, ServerRef, Watcher,
+};
 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::{Addressable, Erased, Name, Pid, RawPid};
-pub use pg::{join, leave, members, pick, Incarnation, Member, NodeId};
+pub use pg::{dispatch, join, leave, members, members_as, pick, pick_as, Incarnation, Member, NodeId};
 pub use registry::{
-    install, register, send, send_dyn, send_to, unregister, whereis, RegisterError, SendError,
+    install, lookup_as, 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,
+    block_on_io, request_stop, run, self_pid, sleep, spawn, spawn_addr, spawn_under, wait_readable,
     wait_readable_timeout, wait_writable, wait_writable_timeout, yield_now, FdArm, JoinError,
     JoinHandle,
 };