bxcore: a turn's speaker, SessionUpdated's turn, the session-state messages

Co-Authored-By: ornith-1.5-35b-a3b <agent@wntrmute.dev>

crates/loopd/tests/turn_done_untrusted.rs

@@ -74,6 +74,7 @@ fn untrusted(socket: &Path, session: &str, content: &str, resume: bool) -> bool
         session: SessionId::new(session).unwrap(),
         content: content.to_string(),
         resume,
+        speaker: None,
     });
     let turn = Envelope {
         v: PROTOCOL_VERSION,