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/logs.rs

@@ -135,11 +135,7 @@ pub struct LogSink {
 }
 
 impl LogSink {
-    pub fn new(
-        server_name: String,
-        writer: RotatingLogWriter,
-        ring_capacity_bytes: usize,
-    ) -> Self {
+    pub fn new(server_name: String, writer: RotatingLogWriter, ring_capacity_bytes: usize) -> Self {
         let (tx, _) = broadcast::channel(LOG_BROADCAST_CAP);
         Self {
             server_name,
@@ -183,8 +179,8 @@ fn now_unix_ms() -> u64 {
 #[cfg(test)]
 mod tests {
     use super::*;
-    use tempfile::tempdir;
     use std::io::Read;
+    use tempfile::tempdir;
 
     #[test]
     fn writes_lines_with_tags() {
@@ -194,10 +190,7 @@ mod tests {
         w.write_line("[out]", "hello").unwrap();
         w.write_line("[err]", "boom").unwrap();
         let mut s = String::new();
-        File::open(&base)
-            .unwrap()
-            .read_to_string(&mut s)
-            .unwrap();
+        File::open(&base).unwrap().read_to_string(&mut s).unwrap();
         assert_eq!(s, "[out] hello\n[err] boom\n");
     }