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>

config.example.toml

@@ -1,5 +1,8 @@
 # artmap configuration
-# Run with: go run . -config config.toml [-artnet-listen :6454]
+# Run with: go run . -config config.toml [-artnet-listen :6454] [-sacn-pcap en0]
+#
+# Use -sacn-pcap <interface> to capture sACN via pcap instead of binding port 5568.
+# This avoids port conflicts but requires root. Use "auto" for auto-detect.
 
 # Target addresses for ArtNet output universes
 # Each output universe needs a target IP (broadcast or unicast)