feat: harden engine config, discovery and backpressure stats

- config: validate port/quality/fps/source/stream-index ranges on load
  and before engine start (flinger.Config.Validate)
- discovery: add Announce option to disable multicast (GUI checkbox,
  CLI --no-announce); absent JSON key keeps the default true
- backpressure: count dropped frames in the TCP sender, expose via
  engine Status and a live counter in the GUI status label
- docs: record M3/M4 decisions and X11 coverage via the portal backend

cmd/teleportfling/main.go

@@ -39,6 +39,7 @@ func main() {
 		fps         = flag.Int("fps", 30, "video frames per second")
 		source      = flag.String("source", "screen", "capture source: screen or pattern")
 		withAudio   = flag.Bool("audio", true, "capture and stream system audio")
+		noAnnounce  = flag.Bool("no-announce", false, "do not announce on the LAN (receiver must connect by IP)")
 		streamIndex = flag.Int("stream-index", 0, "monitor index to capture (screen source)")
 		duration    = flag.Duration("duration", 0, "stream duration (0 = run until interrupted)")
 	)
@@ -52,6 +53,7 @@ func main() {
 		Source:      *source,
 		Audio:       *withAudio,
 		StreamIndex: *streamIndex,
+		Announce:    !*noAnnounce,
 	}
 
 	eng, err := flinger.New(cfg)