session-start and room-joined synthetic notices via generic notices list

src/types.rs

@@ -194,5 +194,8 @@ pub struct DaemonState {
     pub max_history: usize,
     pub session_idle_minutes: u64,
     pub session_max_events: u32,
+    /// Rooms we've auto-joined since their last turn. The dispatcher emits a
+    /// "just joined" notice on the first turn for the room, then removes it.
+    pub just_joined_rooms: std::collections::HashSet<OwnedRoomId>,
 }