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/gui/gui.go

@@ -10,6 +10,7 @@ import (
 	"errors"
 	"log"
 	"strconv"
+	"time"
 
 	"fyne.io/fyne/v2"
 	"fyne.io/fyne/v2/app"
@@ -43,14 +44,16 @@ type App struct {
 	lock bool // serialises start/stop against UI actions
 
 	// UI state.
-	startBtn   *widget.Button
-	statusLab  *widget.Label
-	portEnt    *widget.Entry
-	qualitySel *widget.Select
-	fpsSel     *widget.Select
-	nameEnt    *widget.Entry
-	audioChk   *widget.Check
-	srcSel     *widget.Select
+	startBtn    *widget.Button
+	statusLab   *widget.Label
+	portEnt     *widget.Entry
+	qualitySel  *widget.Select
+	fpsSel      *widget.Select
+	nameEnt     *widget.Entry
+	audioChk    *widget.Check
+	announceChk *widget.Check
+	srcSel      *widget.Select
+	statsDone   chan struct{}
 }
 
 // Run starts the GUI and blocks until the app exits.
@@ -120,6 +123,14 @@ func (g *App) buildUI() {
 	g.audioChk = widget.NewCheck("Capture system audio", nil)
 	g.audioChk.SetChecked(g.cfg.Audio)
 
+	// Announce over multicast.
+	g.announceChk = widget.NewCheck("Announce on LAN", nil)
+	announce := true
+	if g.cfg.Announce != nil {
+		announce = *g.cfg.Announce
+	}
+	g.announceChk.SetChecked(announce)
+
 	// Status label.
 	g.statusLab = widget.NewLabel("Stopped")
 	g.statusLab.Importance = widget.MediumImportance
@@ -136,6 +147,7 @@ func (g *App) buildUI() {
 			{Text: "Quality", Widget: g.qualitySel},
 			{Text: "Frame rate", Widget: g.fpsSel},
 			{Text: "", Widget: g.audioChk},
+			{Text: "", Widget: g.announceChk},
 		},
 	}
 
@@ -168,6 +180,7 @@ func (g *App) start() {
 	quality, _ := atoi(g.qualitySel.Selected)
 	fps, _ := atoi(g.fpsSel.Selected)
 
+	announce := g.announceChk.Checked
 	g.cfg = config.Config{
 		Name:        g.nameEnt.Text,
 		Port:        port,
@@ -176,6 +189,7 @@ func (g *App) start() {
 		Source:      g.srcSel.Selected,
 		Audio:       g.audioChk.Checked,
 		StreamIndex: g.cfg.StreamIndex,
+		Announce:    &announce,
 	}
 	if err := config.Save(g.cfg); err != nil {
 		log.Printf("gui: config save: %v", err)
@@ -194,10 +208,46 @@ func (g *App) start() {
 	g.statusLab.SetText("Streaming (port " + itoa(g.cfg.Port) + ")")
 	g.statusLab.Importance = widget.SuccessImportance
 	g.refresh()
+	g.watchStats()
+}
+
+// watchStats refreshes the status label with live counters while streaming.
+func (g *App) watchStats() {
+	done := make(chan struct{})
+	g.statsDone = done
+	go func() {
+		tick := time.NewTicker(2 * time.Second)
+		defer tick.Stop()
+		for {
+			select {
+			case <-tick.C:
+				if g.eng == nil {
+					return
+				}
+				st := g.eng.Status()
+				g.statusLab.SetText(formatStatus(st))
+			case <-done:
+				return
+			}
+		}
+	}()
+}
+
+// formatStatus renders the live status line.
+func formatStatus(st flinger.Status) string {
+	base := "Streaming · " + itoa(int(st.Frames)) + " frames"
+	if st.Dropped > 0 {
+		base += " · " + itoa(int(st.Dropped)) + " dropped"
+	}
+	return base
 }
 
 // stop halts the engine and returns the UI to the stopped state.
 func (g *App) stop() {
+	if g.statsDone != nil {
+		close(g.statsDone)
+		g.statsDone = nil
+	}
 	if g.eng != nil {
 		g.eng.Stop()
 		g.eng = nil