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/protocol.go

@@ -0,0 +1,102 @@
+package sacn
+
+import (
+	"encoding/binary"
+	"net"
+)
+
+const (
+	Port = 5568
+
+	// ACN packet identifiers
+	ACNPacketIdentifier = 0x41534300 // "ASC\0" + more bytes
+
+	// Vector values
+	VectorRootE131Data    = 0x00000004
+	VectorE131DataPacket  = 0x00000002
+	VectorDMPSetProperty  = 0x02
+)
+
+var (
+	// ACN packet identifier (12 bytes)
+	packetIdentifier = [12]byte{
+		0x41, 0x53, 0x43, 0x2d, 0x45, 0x31, 0x2e, 0x31, 0x37, 0x00, 0x00, 0x00,
+	}
+)
+
+// BuildDataPacket creates an E1.31 (sACN) data packet
+func BuildDataPacket(universe uint16, sequence uint8, sourceName string, cid [16]byte, data []byte) []byte {
+	dataLen := len(data)
+	if dataLen > 512 {
+		dataLen = 512
+	}
+
+	// Total packet size: Root Layer (38) + Framing Layer (77) + DMP Layer (11 + data)
+	// = 126 + dataLen
+	pktLen := 126 + dataLen
+	buf := make([]byte, pktLen)
+
+	// Root Layer (38 bytes)
+	// Preamble Size (2 bytes)
+	binary.BigEndian.PutUint16(buf[0:2], 0x0010)
+	// Post-amble Size (2 bytes)
+	binary.BigEndian.PutUint16(buf[2:4], 0x0000)
+	// ACN Packet Identifier (12 bytes)
+	copy(buf[4:16], packetIdentifier[:])
+	// Flags and Length (2 bytes) - high 4 bits are flags (0x7), low 12 bits are length
+	rootLen := pktLen - 16 // Length from after ACN Packet Identifier
+	binary.BigEndian.PutUint16(buf[16:18], 0x7000|uint16(rootLen))
+	// Vector (4 bytes)
+	binary.BigEndian.PutUint32(buf[18:22], VectorRootE131Data)
+	// CID (16 bytes)
+	copy(buf[22:38], cid[:])
+
+	// Framing Layer (77 bytes, starting at offset 38)
+	// Flags and Length (2 bytes)
+	framingLen := pktLen - 38
+	binary.BigEndian.PutUint16(buf[38:40], 0x7000|uint16(framingLen))
+	// Vector (4 bytes)
+	binary.BigEndian.PutUint32(buf[40:44], VectorE131DataPacket)
+	// Source Name (64 bytes, null-terminated)
+	copy(buf[44:108], sourceName)
+	// Priority (1 byte)
+	buf[108] = 100
+	// Synchronization Address (2 bytes)
+	binary.BigEndian.PutUint16(buf[109:111], 0)
+	// Sequence Number (1 byte)
+	buf[111] = sequence
+	// Options (1 byte)
+	buf[112] = 0
+	// Universe (2 bytes)
+	binary.BigEndian.PutUint16(buf[113:115], universe)
+
+	// DMP Layer (11 + dataLen bytes, starting at offset 115)
+	// Flags and Length (2 bytes)
+	dmpLen := 11 + dataLen
+	binary.BigEndian.PutUint16(buf[115:117], 0x7000|uint16(dmpLen))
+	// Vector (1 byte)
+	buf[117] = VectorDMPSetProperty
+	// Address Type & Data Type (1 byte)
+	buf[118] = 0xa1
+	// First Property Address (2 bytes)
+	binary.BigEndian.PutUint16(buf[119:121], 0)
+	// Address Increment (2 bytes)
+	binary.BigEndian.PutUint16(buf[121:123], 1)
+	// Property Value Count (2 bytes) - includes START code
+	binary.BigEndian.PutUint16(buf[123:125], uint16(dataLen+1))
+	// START Code (1 byte)
+	buf[125] = 0
+	// Property Values (DMX data)
+	copy(buf[126:], data[:dataLen])
+
+	return buf
+}
+
+// MulticastAddr returns the multicast address for a given universe
+func MulticastAddr(universe uint16) *net.UDPAddr {
+	// 239.255.{universe_high}.{universe_low}
+	return &net.UDPAddr{
+		IP:   net.IPv4(239, 255, byte(universe>>8), byte(universe&0xff)),
+		Port: Port,
+	}
+}