Add --artnet-broadcast flag for smarter broadcast handling

- Accepts comma-separated list of addresses or 'auto'
- Auto-detection calculates broadcast from all network interfaces
- Used as fallback when no per-universe target or discovered nodes

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

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

config.example.toml

@@ -1,8 +1,10 @@
 # artmap configuration
-# Run with: go run . -config config.toml [-artnet-listen :6454] [-sacn-pcap en0]
+# Run with: go run . --config=config.toml [flags]
 #
-# 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.
+# Flags:
+#   --artnet-listen=:6454        ArtNet listen address (empty to disable)
+#   --artnet-broadcast=auto      Broadcast addresses (comma-separated, or 'auto')
+#   --sacn-pcap=en0              Use pcap for sACN (requires root, avoids port conflicts)
 
 # Target addresses for ArtNet output universes
 # Each output universe needs a target IP (broadcast or unicast)