feat(supervisor): supervisor task with state machine

One async task per managed server owns all state transitions via a
tokio::select! loop over cmd_rx and wait_child. Includes RealSpawner
and a smoke test covering the Start → Running → exit → Stopped →
Shutdown happy path.

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

crates/xy-supervisor/src/lib.rs

@@ -5,9 +5,13 @@ pub mod child;
 pub mod logs;
 pub mod policy;
 pub mod retry_window;
+pub mod supervisor;
 
 pub use backoff::Backoff;
 pub use child::{ChildHandle, MockChild, MockChildController, RealChild, spawn_with_logs};
 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,
+};