fix(supervisor): compute uptime at read time, not publish time

set_state computed uptime_secs from started_at.elapsed() and pushed the
result into the watch channel. do_start sets started_at immediately before
calling set_state(Running), so the published value was always ~0, and the
list/status handlers copied that frozen snapshot straight out. A server
that stayed healthy reported 0s forever, because the value was only
recomputed on the next state transition.

Status now carries started_at (an Instant, in-process only) and the
handlers derive uptime_secs when they build ServerSummary. The wire type
is unchanged.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EGntTHCW3sEPy1VBRopNNp

crates/xy-supervisor/src/supervisor.rs

@@ -48,7 +48,7 @@ pub struct Status {
     pub state: ServerState,
     pub pid: Option<u32>,
     pub port: u16,
-    pub uptime_secs: Option<u64>,
+    pub started_at: Option<Instant>,
     pub restart_count: u32,
     pub last_exit: Option<i32>,
 }
@@ -109,13 +109,11 @@ impl<S: Spawner> SupervisorTask<S> {
     }
 
     fn set_state(&mut self, s: ServerState) {
-        let uptime_secs = self.started_at.map(|t| t.elapsed().as_secs());
-
         let _ = self.status_tx.send(Status {
             state: s,
             pid: self.current_pid,
             port: self.cfg.port,
-            uptime_secs,
+            started_at: self.started_at,
             restart_count: self.restart_count,
             last_exit: self.last_exit,
         });
@@ -407,7 +405,7 @@ mod tests {
             state: ServerState::Stopped,
             pid: None,
             port: cfg.port,
-            uptime_secs: None,
+            started_at: None,
             restart_count: 0,
             last_exit: None,
         }