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/session.rs

@@ -62,6 +62,7 @@ fn append_is_visible_on_disk_at_once_and_after_reopen() {
     s.append(LogRecord::User {
         time: ts(),
         content: "hi\nthere \"quoted\" caf\u{e9}".to_string(),
+        speaker: None,
     })
     .unwrap();
     assert_eq!(
@@ -104,6 +105,7 @@ fn a_torn_log_is_refused_with_the_line_number() {
     s.append(LogRecord::User {
         time: ts(),
         content: "hi".to_string(),
+        speaker: None,
     })
     .unwrap();
     drop(s);
@@ -173,6 +175,7 @@ fn last_usage_is_the_latest_usage_record() {
     s.append(LogRecord::User {
         time: ts(),
         content: "x".to_string(),
+        speaker: None,
     })
     .unwrap();
     s.append(LogRecord::Usage {