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

internal/flinger/flinger.go

@@ -48,23 +48,48 @@ type Config struct {
 	Audio bool
 	// StreamIndex selects which monitor to capture (screen source only).
 	StreamIndex int
+	// Announce controls whether the stream is advertised via UDP multicast.
+	// When disabled, receivers must connect by IP manually.
+	Announce bool
 }
 
 // DefaultConfig returns the recommended defaults.
 func DefaultConfig() Config {
 	return Config{
-		Port:    9756,
-		Quality: 80,
-		FPS:     30,
-		Source:  "screen",
-		Audio:   true,
+		Port:     9756,
+		Quality:  80,
+		FPS:      30,
+		Source:   "screen",
+		Audio:    true,
+		Announce: true,
 	}
 }
 
+// Validate checks the config for out-of-range or unsupported values.
+func (c Config) Validate() error {
+	if c.Port < 1 || c.Port > 65535 {
+		return errors.New("port must be 1–65535")
+	}
+	if c.Quality < 1 || c.Quality > 100 {
+		return errors.New("quality must be 1–100")
+	}
+	if c.FPS < 1 || c.FPS > 240 {
+		return errors.New("fps must be 1–240")
+	}
+	if c.Source != "screen" && c.Source != "pattern" {
+		return errors.New("source must be screen or pattern")
+	}
+	if c.StreamIndex < 0 {
+		return errors.New("stream index must be >= 0")
+	}
+	return nil
+}
+
 // Status is a point-in-time snapshot of the running engine.
 type Status struct {
 	Running bool
 	Frames  int64
+	Dropped int64
 	Conns   int
 }
 
@@ -87,6 +112,10 @@ type Engine struct {
 // New creates an engine from cfg. Capture is opened eagerly so that
 // misconfiguration (e.g. no screen-share permission) surfaces before Start.
 func New(cfg Config) (*Engine, error) {
+	if err := cfg.Validate(); err != nil {
+		return nil, err
+	}
+
 	e := &Engine{cfg: cfg}
 
 	sender := output.New()
@@ -124,8 +153,8 @@ func New(cfg Config) (*Engine, error) {
 	return e, nil
 }
 
-// Start begins the audio, video and stats loops and starts announcing the
-// stream. It is idempotent.
+// Start begins the audio, video and stats loops and (unless disabled in the
+// config) starts announcing the stream. It is idempotent.
 func (e *Engine) Start() {
 	if e.stop != nil {
 		return
@@ -134,7 +163,9 @@ func (e *Engine) Start() {
 	e.start = time.Now()
 	e.stop = make(chan struct{})
 
-	e.announcer = discovery.Start(e.cfg.Name, e.sender.Port())
+	if e.cfg.Announce {
+		e.announcer = discovery.Start(e.cfg.Name, e.sender.Port())
+	}
 
 	var src io.ReadCloser
 	switch {
@@ -180,6 +211,7 @@ func (e *Engine) Status() Status {
 	return Status{
 		Running: e.stop != nil,
 		Frames:  e.frames.Load(),
+		Dropped: e.sender.Dropped(),
 		Conns:   e.sender.NumConns(),
 	}
 }
@@ -276,7 +308,11 @@ func (e *Engine) statsLoop() {
 		select {
 		case <-tick.C:
 			st := e.Status()
-			log.Printf("flinger: %d frames, %d conns", st.Frames, st.Conns)
+			if st.Dropped > 0 {
+				log.Printf("flinger: %d frames, %d dropped, %d conns", st.Frames, st.Dropped, st.Conns)
+			} else {
+				log.Printf("flinger: %d frames, %d conns", st.Frames, st.Conns)
+			}
 		case <-e.stop:
 			return
 		}

internal/flinger/flinger_test.go

@@ -11,7 +11,7 @@ import (
 func TestEnginePatternStartStop(t *testing.T) {
 	cfg := DefaultConfig()
 	cfg.Source = "pattern"
-	cfg.Port = 0 // ephemeral
+	cfg.Port = 19756 // fixed high port for the test
 
 	eng, err := New(cfg)
 	if err != nil {
@@ -77,6 +77,29 @@ func TestNewRejectsBadSource(t *testing.T) {
 	}
 }
 
+// TestValidate rejects out-of-range values.
+func TestValidate(t *testing.T) {
+	bad := []func(*Config){
+		func(c *Config) { c.Port = 0 },
+		func(c *Config) { c.Port = 70000 },
+		func(c *Config) { c.Quality = 0 },
+		func(c *Config) { c.Quality = 101 },
+		func(c *Config) { c.FPS = 0 },
+		func(c *Config) { c.StreamIndex = -1 },
+	}
+	for i, mutate := range bad {
+		c := DefaultConfig()
+		mutate(&c)
+		if err := c.Validate(); err == nil {
+			t.Errorf("case %d: expected validation error", i)
+		}
+	}
+
+	if err := DefaultConfig().Validate(); err != nil {
+		t.Errorf("default config should validate: %v", err)
+	}
+}
+
 func itoa(v int) string {
 	if v == 0 {
 		return "0"