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/output/sender.go

@@ -16,6 +16,7 @@ import (
 	"log"
 	"net"
 	"sync"
+	"sync/atomic"
 )
 
 const (
@@ -34,6 +35,8 @@ type Sender struct {
 
 	listener net.Listener
 	port     int
+
+	dropped atomic.Int64
 }
 
 // New creates an unconnected Sender.
@@ -86,7 +89,8 @@ func (s *Sender) NumConns() int {
 
 // Send broadcasts a serialised packet to all connected receivers. If a
 // receiver's buffered channel is full (> dropAt) the frame is silently
-// dropped; a warning is logged at warnAt.
+// dropped; a warning is logged at warnAt. Dropped frames are counted and
+// exposed via Dropped.
 func (s *Sender) Send(b []byte) {
 	s.mu.Lock()
 	defer s.mu.Unlock()
@@ -101,6 +105,7 @@ func (s *Sender) Send(b []byte) {
 		}
 
 		if len(ch) > dropAt {
+			s.dropped.Add(1)
 			continue
 		}
 
@@ -109,11 +114,17 @@ func (s *Sender) Send(b []byte) {
 		select {
 		case ch <- b:
 		default:
+			s.dropped.Add(1)
 			log.Printf("output: drop [%s] (queue full)", c.RemoteAddr())
 		}
 	}
 }
 
+// Dropped returns the total number of frames dropped due to backpressure.
+func (s *Sender) Dropped() int64 {
+	return s.dropped.Load()
+}
+
 // Close shuts down the listener and waits for all writer goroutines to
 // drain. After Close returns the Sender must not be reused.
 func (s *Sender) Close() {