feat: implement M1 teleport protocol sender

Stream a synthetic test pattern and silent PCM audio over the OBS
Teleport protocol:
- protocol: wire format (Header/ImageHeader/WaveHeader), BT.709 full
  range colour matrix, JPEG encode via turbojpeg cgo, WAVE packet builder
- output: TCP sender with per-connection buffered channels and drop-on-overflow
- discovery: multicast announce via peerdiscovery
- cmd: teleportfling CLI with flags, test-pattern frame generator

Verified end-to-end: OBS discovers and renders the stream with correct
colours and motion.

internal/output/sender.go

@@ -0,0 +1,179 @@
+// Package output implements the TCP sender and receiver-connection manager
+// for the Teleport protocol. It mirrors the design of obs-teleport's
+// Sender: a per-connection buffered channel (capacity 1000), frame drop
+// when the queue exceeds 800, and a warning at 100.
+//
+// Usage:
+//
+//	s := output.New()
+//	s.Listen(":9756")
+//	s.Send(packet)           // from any goroutine
+//	...
+//	s.Close()
+package output
+
+import (
+	"log"
+	"net"
+	"sync"
+)
+
+const (
+	sendChanCap = 1000
+	dropAt      = 800
+	warnAt      = 100
+)
+
+// Sender manages a TCP listener and a set of connected receivers.
+// Frames are distributed to every connected receiver; a receiver whose
+// queue grows too large has its next frame silently dropped.
+type Sender struct {
+	mu    sync.Mutex
+	conns map[net.Conn]chan []byte
+	wg    sync.WaitGroup
+
+	listener net.Listener
+	port     int
+}
+
+// New creates an unconnected Sender.
+func New() *Sender {
+	return &Sender{
+		conns: make(map[net.Conn]chan []byte),
+	}
+}
+
+// Listen binds a TCP listener on addr (e.g. ":9756") and starts accepting
+// connections. It returns the resolved port number on success. The caller
+// must eventually call Close.
+func (s *Sender) Listen(addr string) (int, error) {
+	l, err := net.Listen("tcp", addr)
+	if err != nil {
+		return 0, err
+	}
+	s.listener = l
+
+	// Extract the port so the caller can advertise it via UDP announce.
+	_, p, err := net.SplitHostPort(l.Addr().String())
+	if err != nil {
+		if cerr := l.Close(); cerr != nil {
+			log.Printf("output: close failed: %v", cerr)
+		}
+		return 0, err
+	}
+
+	var port int
+	for _, ch := range p {
+		port = port*10 + int(ch-'0')
+	}
+	s.port = port
+
+	s.wg.Add(1)
+	go s.acceptLoop()
+
+	return port, nil
+}
+
+// Port returns the TCP port that was assigned at Listen time.
+func (s *Sender) Port() int { return s.port }
+
+// NumConns returns the number of connected receivers.
+func (s *Sender) NumConns() int {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	return len(s.conns)
+}
+
+// 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.
+func (s *Sender) Send(b []byte) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
+	for c, ch := range s.conns {
+		switch {
+		case len(ch) > dropAt:
+			log.Printf("output: drop [%s] (queue %d)", c.RemoteAddr(), len(ch))
+		case len(ch) > warnAt:
+			log.Printf("output: high [%s] (queue %d)", c.RemoteAddr(), len(ch))
+		default:
+		}
+
+		if len(ch) > dropAt {
+			continue
+		}
+
+		// Non-blocking send: if the channel is full after the drop
+		// window, drop the frame.
+		select {
+		case ch <- b:
+		default:
+			log.Printf("output: drop [%s] (queue full)", c.RemoteAddr())
+		}
+	}
+}
+
+// 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() {
+	if s.listener != nil {
+		// Best-effort close; the accept loop will observe the listener error.
+		_ = s.listener.Close()
+	}
+
+	s.mu.Lock()
+	for _, ch := range s.conns {
+		close(ch)
+	}
+	s.conns = nil
+	s.mu.Unlock()
+
+	s.wg.Wait()
+}
+
+// acceptLoop runs in its own goroutine and adds incoming connections.
+func (s *Sender) acceptLoop() {
+	defer s.wg.Done()
+	for {
+		c, err := s.listener.Accept()
+		if err != nil {
+			return
+		}
+		s.SenderAdd(c)
+	}
+}
+
+// SenderAdd registers a new receiver connection and spawns its writer.
+func (s *Sender) SenderAdd(c net.Conn) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
+	log.Printf("output: connect %s", c.RemoteAddr())
+
+	ch := make(chan []byte, sendChanCap)
+	s.conns[c] = ch
+
+	s.wg.Add(1)
+	go func() {
+		defer s.wg.Done()
+		defer func() {
+			// Best-effort close; writer loop also closes on write error.
+			_ = c.Close()
+		}()
+
+		for b := range ch {
+			if _, err := c.Write(b); err != nil {
+				log.Printf("output: disconnect %s", c.RemoteAddr())
+				s.removeConn(c)
+				return
+			}
+		}
+	}()
+}
+
+func (s *Sender) removeConn(c net.Conn) {
+	s.mu.Lock()
+	delete(s.conns, c)
+	s.mu.Unlock()
+}