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/config/config.go

@@ -23,11 +23,15 @@ type Config struct {
 	Source      string `json:"source"`
 	Audio       bool   `json:"audio"`
 	StreamIndex int    `json:"stream_index"`
+	// Announce is a *bool so an absent JSON key (older config files) keeps
+	// the default instead of silently disabling announcements.
+	Announce *bool `json:"announce"`
 }
 
 // Default returns the default configuration.
 func Default() Config {
 	c := flinger.DefaultConfig()
+	announce := c.Announce
 	return Config{
 		Name:        c.Name,
 		Port:        c.Port,
@@ -36,11 +40,16 @@ func Default() Config {
 		Source:      c.Source,
 		Audio:       c.Audio,
 		StreamIndex: c.StreamIndex,
+		Announce:    &announce,
 	}
 }
 
 // ToFlinger converts a persisted config to the engine config.
 func (c Config) ToFlinger() flinger.Config {
+	announce := true
+	if c.Announce != nil {
+		announce = *c.Announce
+	}
 	return flinger.Config{
 		Name:        c.Name,
 		Port:        c.Port,
@@ -49,6 +58,7 @@ func (c Config) ToFlinger() flinger.Config {
 		Source:      c.Source,
 		Audio:       c.Audio,
 		StreamIndex: c.StreamIndex,
+		Announce:    announce,
 	}
 }
 
@@ -96,6 +106,10 @@ func Load() (Config, error) {
 	if c.Source == "" {
 		c.Source = d.Source
 	}
+	if c.Announce == nil {
+		announce := true
+		c.Announce = &announce
+	}
 	return c, nil
 }