fix(supervisor): StartAck::SpawnFailed surfaces real failures

Add StartAck::SpawnFailed(String) so callers can distinguish a successful
start from a failed spawn. The Start command arm now sends SpawnFailed on
io::Error rather than the misleading Started. handlers.rs maps the new
variant to an RpcErrorCode::SpawnFailed JSON-RPC error response.

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

crates/xy-supervisor/src/supervisor.rs

@@ -34,6 +34,7 @@ pub enum SupervisorCmd {
 pub enum StartAck {
     Started,
     AlreadyRunning,
+    SpawnFailed(String),
 }
 
 #[derive(Debug, PartialEq, Eq)]
@@ -141,7 +142,7 @@ impl<S: Spawner> SupervisorTask<S> {
                                     Err(err) => {
                                         warn!(name = %self.cfg.name, error = %err, "spawn failed");
                                         self.set_state(ServerState::Failed);
-                                        let _ = ack.send(StartAck::Started);
+                                        let _ = ack.send(StartAck::SpawnFailed(err.to_string()));
                                     }
                                 }
                             }

crates/xy/src/daemon/handlers.rs

@@ -247,6 +247,13 @@ async fn dispatch_lifecycle(
                 match rx.await {
                     Ok(StartAck::Started) => started.push(name),
                     Ok(StartAck::AlreadyRunning) => already.push(name),
+                    Ok(StartAck::SpawnFailed(msg)) => {
+                        return err_response(
+                            id,
+                            RpcErrorCode::SpawnFailed.as_i32(),
+                            format!("failed to start `{name}`: {msg}"),
+                        );
+                    }
                     Err(_) => {
                         return err_response(
                             id,