Add sACN input/output support and fix multi-source merging

- Add sACN/E1.31 protocol support for both input and output
- from_proto = "sacn" to receive from sACN multicast
- proto = "sacn" to output via sACN multicast
- Fix remap engine to maintain persistent state per output universe
- Multiple inputs targeting same output now merge correctly
- Prevents flickering when multiple universes feed same output

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

sacn/receiver.go

@@ -0,0 +1,140 @@
+package sacn
+
+import (
+	"encoding/binary"
+	"log"
+	"net"
+
+	"golang.org/x/net/ipv4"
+)
+
+// DMXHandler is called when DMX data is received
+type DMXHandler func(universe uint16, data [512]byte)
+
+// Receiver listens for sACN packets
+type Receiver struct {
+	conn      *ipv4.PacketConn
+	universes []uint16
+	handler   DMXHandler
+	done      chan struct{}
+}
+
+// NewReceiver creates a new sACN receiver for the given universes
+func NewReceiver(universes []uint16, handler DMXHandler) (*Receiver, error) {
+	// Listen on sACN port
+	c, err := net.ListenPacket("udp4", ":5568")
+	if err != nil {
+		return nil, err
+	}
+
+	p := ipv4.NewPacketConn(c)
+
+	// Join multicast groups for each universe
+	for _, u := range universes {
+		group := net.IPv4(239, 255, byte(u>>8), byte(u&0xff))
+		iface, _ := net.InterfaceByIndex(0) // Use default interface
+		if err := p.JoinGroup(iface, &net.UDPAddr{IP: group}); err != nil {
+			// Try joining on all interfaces
+			ifaces, _ := net.Interfaces()
+			for _, iface := range ifaces {
+				p.JoinGroup(&iface, &net.UDPAddr{IP: group})
+			}
+		}
+	}
+
+	return &Receiver{
+		conn:      p,
+		universes: universes,
+		handler:   handler,
+		done:      make(chan struct{}),
+	}, nil
+}
+
+// Start begins receiving packets
+func (r *Receiver) Start() {
+	go r.receiveLoop()
+}
+
+// Stop stops the receiver
+func (r *Receiver) Stop() {
+	close(r.done)
+	r.conn.Close()
+}
+
+func (r *Receiver) receiveLoop() {
+	buf := make([]byte, 638) // Max sACN packet size
+
+	for {
+		select {
+		case <-r.done:
+			return
+		default:
+		}
+
+		n, _, _, err := r.conn.ReadFrom(buf)
+		if err != nil {
+			select {
+			case <-r.done:
+				return
+			default:
+				log.Printf("sacn read error: %v", err)
+				continue
+			}
+		}
+
+		r.handlePacket(buf[:n])
+	}
+}
+
+func (r *Receiver) handlePacket(data []byte) {
+	// Minimum packet size check
+	if len(data) < 126 {
+		return
+	}
+
+	// Check ACN packet identifier
+	if data[4] != 0x41 || data[5] != 0x53 || data[6] != 0x43 {
+		return
+	}
+
+	// Check root vector (E1.31 data)
+	rootVector := binary.BigEndian.Uint32(data[18:22])
+	if rootVector != VectorRootE131Data {
+		return
+	}
+
+	// Check framing vector (DMP data)
+	framingVector := binary.BigEndian.Uint32(data[40:44])
+	if framingVector != VectorE131DataPacket {
+		return
+	}
+
+	// Get universe
+	universe := binary.BigEndian.Uint16(data[113:115])
+
+	// Check DMP vector
+	if data[117] != VectorDMPSetProperty {
+		return
+	}
+
+	// Get property count (includes START code)
+	propCount := binary.BigEndian.Uint16(data[123:125])
+	if propCount < 1 {
+		return
+	}
+
+	// Skip START code at data[125]
+	dmxLen := int(propCount) - 1
+	if dmxLen > 512 {
+		dmxLen = 512
+	}
+
+	if len(data) < 126+dmxLen {
+		return
+	}
+
+	var dmxData [512]byte
+	copy(dmxData[:], data[126:126+dmxLen])
+
+	r.handler(universe, dmxData)
+}