Initial implementation of mc-proxy

Layer 4 TLS SNI proxy with global firewall (IP/CIDR/GeoIP blocking),
per-listener route tables, bidirectional TCP relay with half-close
propagation, and a gRPC admin API (routes, firewall, status) with
TLS/mTLS support.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.gitignore

@@ -0,0 +1,12 @@
+# Binary
+mc-proxy
+
+# Runtime data
+srv/
+
+# IDE
+.idea/
+.vscode/
+
+# OS
+.DS_Store