feat: Docker Compose E2E test env, agent fixes, TAP MAC config, testclient binary

- Add docker-compose.yml with server + init + 3 clients + tester
- Add Dockerfile.server (CGO multi-stage) and Dockerfile.client
- Add .dockerignore for efficient builds
- Add cmd/testclient/main.go: full E2E test binary using SDK
- Fix agent handshake loop: only respond to new handshakes
- Add tapInterface.SetMAC() to match agent's generated MAC
- Call SetMAC() in agent SetTAPIP to fix ARP resolution
- Add data-plane debug logging for troubleshooting
- Change config database path to /tmp/data/ for container use
- Add SetMAC stub for non-Linux builds

config.yaml

@@ -6,7 +6,7 @@ database:
   auto_migrate: true
   init_data: true
   sqlite:
-    path: ./data/zeromesh.db
+    path: /tmp/data/zeromesh.db
   mysql:
     host: 127.0.0.1
     port: 3306