gatewayd: a turn's speaker, the username of the one person who wrote its burst

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

crates/gatewayd/tests/deliver_links.rs

@@ -68,6 +68,7 @@ fn posts(tag: &str, reply: fn(&str) -> Reply, content: &'static str) -> Vec<Stri
         },
         resume: false,
         text: "hi".to_string(),
+        senders: vec!["k0000000000000000000000000".to_string()],
     };
     let never = |_: u64| false;
     let notice = Notice {
@@ -80,6 +81,7 @@ fn posts(tag: &str, reply: fn(&str) -> Reply, content: &'static str) -> Vec<Stri
         &record,
         &socket,
         &batch,
+        None,
         &|line| log.lock().unwrap().push(line.to_string()),
         &notice,
     );