Replace --broadcast flag with per-universe target config

- Remove --broadcast CLI flag
- Add [[target]] config section for per-universe destination addresses
- Each ArtNet output universe can have its own target IP (broadcast or unicast)
- ArtPoll discovery broadcasts to all unique target addresses
- Falls back to configured target when no nodes discovered for universe

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

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

config.example.toml

@@ -1,5 +1,16 @@
 # artmap configuration
-# Run with: go run . -config config.toml [-listen :6454] [-broadcast 2.255.255.255]
+# Run with: go run . -config config.toml [-listen :6454]
+
+# Target addresses for ArtNet output universes
+# Configure the destination IP (broadcast or unicast) for each output universe
+# ArtPoll discovery will be sent to all unique target addresses
+[[target]]
+universe = "0.0.0"
+address = "2.255.255.255"
+
+[[target]]
+universe = "0.0.5"
+address = "10.50.255.255"
 
 # Address format:
 #   from: universe[:channels]  - range specifies which channels to read