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>

go.mod

@@ -2,4 +2,9 @@ module github.com/gopatchy/artmap
 
 go 1.25.4
 
-require github.com/BurntSushi/toml v1.6.0
+require (
+	github.com/BurntSushi/toml v1.6.0
+	golang.org/x/net v0.48.0
+)
+
+require golang.org/x/sys v0.39.0 // indirect