Add pcap-based sACN receiver to avoid port conflicts

New -sacn-pcap flag uses packet capture instead of binding port 5568.
This allows running alongside other software using the sACN port.

- Captures both incoming and outgoing packets (same-host sender support)
- Requires root/sudo for packet capture privileges
- Use "auto" for automatic interface detection

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

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

go.mod

@@ -4,6 +4,7 @@ go 1.25.4
 
 require (
 	github.com/BurntSushi/toml v1.6.0
+	github.com/google/gopacket v1.1.19
 	golang.org/x/net v0.48.0
 )