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_test.go

@@ -29,6 +29,7 @@ func TestSaveLoadRoundTrip(t *testing.T) {
 	pathVar = filepath.Join(t.TempDir(), "config.json")
 	defer func() { pathVar = old }()
 
+	announce := false
 	want := Config{
 		Name:        "Studio",
 		Port:        9898,
@@ -37,6 +38,7 @@ func TestSaveLoadRoundTrip(t *testing.T) {
 		Source:      "pattern",
 		Audio:       false,
 		StreamIndex: 1,
+		Announce:    &announce,
 	}
 	if err := Save(want); err != nil {
 		t.Fatalf("Save: %v", err)
@@ -46,7 +48,10 @@ func TestSaveLoadRoundTrip(t *testing.T) {
 	if err != nil {
 		t.Fatalf("Load: %v", err)
 	}
-	if got != want {
+	if got.Name != want.Name || got.Port != want.Port || got.Quality != want.Quality ||
+		got.FPS != want.FPS || got.Source != want.Source || got.Audio != want.Audio ||
+		got.StreamIndex != want.StreamIndex || got.Announce == nil || want.Announce == nil ||
+		*got.Announce != *want.Announce {
 		t.Errorf("round trip mismatch:\n got %+v\nwant %+v", got, want)
 	}
 }
@@ -74,4 +79,16 @@ func TestLoadFillsZeroValues(t *testing.T) {
 	if c.Quality != 80 {
 		t.Errorf("quality = %d, want filled default 80", c.Quality)
 	}
+	// Absent "announce" key must default to true (not false).
+	if c.Announce == nil || !*c.Announce {
+		t.Errorf("announce = %v, want default true", c.Announce)
+	}
+}
+
+// TestToFlingerAnnounceDefault checks the announce default survives conversion.
+func TestToFlingerAnnounceDefault(t *testing.T) {
+	c := Config{}
+	if !c.ToFlinger().Announce {
+		t.Error("ToFlinger announce default should be true")
+	}
 }