feat(gen_server): handle_down — runtime monitor forwarding via ServerCtx/Watcher

Monitors are created at runtime (you watch a worker you just spawned in a
handler), so down arms can't ride the static info list. init grows a
&ServerCtx parameter (breaking; no-op default) whose clonable Watcher hands
Monitors to the loop over a control arm; the loop selects the live down
arms ahead of everything else. Arm priority: downs → control → infos →
inbox. A delivered Down retires its arm (monitors are one-shot); a state
that never clones the Watcher closes the control arm after init and the
loop falls back to the plain-inbox park.

src/lib.rs

@@ -46,7 +46,7 @@ static ALLOCATOR: preempt::PreemptingAllocator = preempt::PreemptingAllocator;
 // ---------------------------------------------------------------------------
 
 pub use channel::{channel, select, select_timeout, Receiver, RecvError, RecvTimeoutError, Selectable, Sender};
-pub use gen_server::{CallError, CallTimeoutError, CastError, GenServer, ServerBuilder, ServerRef};
+pub use gen_server::{CallError, CallTimeoutError, CastError, GenServer, ServerBuilder, ServerCtx, 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};