WIP: streamer Start takes a ctx; README and TODO

Model: opus-5-5

cmd/streamdumper/main.go

@@ -31,7 +31,7 @@ func main() {
 
 	// Start streaming before any defer: log.Fatal exits without running
 	// deferred calls.
-	err := s.Start()
+	err := s.Start(context.Background())
 	if err != nil {
 		log.Fatal("Failed to start streamer:", err)
 	}