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/view.go

@@ -354,6 +354,8 @@ func (m Model) routerRows(w, h int) []string {
 			}
 		case "tool":
 			rows = append(rows, t.span("· tool output ("+itoaLen(e.Content)+" chars) — ^x to view", t.P.Dim))
+		case "stage":
+			rows = append(rows, t.span(e.Content, t.P.Faint))
 		}
 	}
 	// keep last h rows