feat(tui): render stage lifecycle frames as dim lines in the router feed

Stage started/completed/failed events are now injected into the router
transcript (role "stage") and rendered in Faint colour, giving the
operator a chronological in-feed view of workflow progress alongside
user/router/tool turns.

apps/tui-go/internal/app/server.go

@@ -102,16 +102,19 @@ func (m *Model) applyServer(msg protocol.ServerMessage) {
 			s.Tools = nil
 			s.addEvent(msg.OccurredAt, "StageStarted", msg.StageID)
 		}
+		m.appendRouter(msg.SessionID, RouterEntry{"stage", "⟐ stage " + msg.StageID + " started"})
 	case protocol.TypeStageCompleted:
 		if s := m.session(msg.SessionID); s != nil {
 			s.CurrentStage = ""
 			s.addEvent(msg.OccurredAt, "StageCompleted", msg.StageID)
 		}
+		m.appendRouter(msg.SessionID, RouterEntry{"stage", "⟐ stage " + msg.StageID + " completed"})
 	case protocol.TypeStageFailed:
 		if s := m.session(msg.SessionID); s != nil {
 			s.CurrentStage = ""
 			s.addEvent(msg.OccurredAt, "StageFailed", msg.StageID)
 		}
+		m.appendRouter(msg.SessionID, RouterEntry{"stage", "⟐ stage " + msg.StageID + " failed"})
 	case protocol.TypeInferenceStarted:
 		if s := m.session(msg.SessionID); s != nil {
 			s.Active = true