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

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

crates/bxctl/src/chat.rs

@@ -87,6 +87,8 @@ pub fn run_turn(
         session: session.clone(),
         content: content.to_string(),
         resume,
+        // Set in M5c-1 task 04, from `--speaker`. Until then no turn has a speaker.
+        speaker: None,
     };
     write_frame(
         &mut stream,

crates/bxctl/tests/chat.rs

@@ -174,7 +174,8 @@ fn run_turn_delivers_every_event_in_order_then_the_answer() {
         Turn {
             session: id("s1"),
             content: "what time is it?".to_string(),
-            resume: false
+            resume: false,
+            speaker: None,
         }
     );
 }