fix(supervisor): publish full status (pid, port, uptime, restart_count, last_exit) via watch channel

Replace watch::Receiver<ServerState> on SupervisorHandle with watch::Receiver<Status>,
a richer snapshot type that carries pid, port, uptime_secs, restart_count and last_exit.
SupervisorTask maintains current_pid and publishes a fresh Status on every state
transition; handlers.rs reads the full Status so list/status no longer return
zeroed/None fields.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

crates/xy-supervisor/src/lib.rs

@@ -13,5 +13,6 @@ pub use logs::{LogSink, RecordedLine, RingBuffer, RotatingLogWriter};
 pub use policy::{RestartDecision, decide};
 pub use retry_window::RetryWindow;
 pub use supervisor::{
-    RealSpawner, Spawner, StartAck, StopAck, SupervisorCmd, SupervisorHandle, SupervisorTask,
+    RealSpawner, Spawner, StartAck, Status, StopAck, SupervisorCmd, SupervisorHandle,
+    SupervisorTask,
 };