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

sdk/tap_linux.go

@@ -4,6 +4,7 @@ package sdk
 
 import (
 	"fmt"
+	"os/exec"
 	"syscall"
 	"unsafe"
 )
@@ -35,7 +36,6 @@ func openTap(name string, mtu int) (*tapInterface, error) {
 	}
 	ti := &tapInterface{Name: devName, MTU: mtu, fd: fd}
 
-	// Set MTU via socket ioctl
 	s, err := syscall.Socket(syscall.AF_INET, syscall.SOCK_DGRAM, 0)
 	if err == nil {
 		defer syscall.Close(s)
@@ -45,7 +45,7 @@ func openTap(name string, mtu int) (*tapInterface, error) {
 		}
 		copy(mtuReq.name[:], []byte(devName))
 		mtuReq.mtu = int32(mtu)
-		syscall.Syscall(syscall.SYS_IOCTL, uintptr(s), 0x400c4a52, uintptr(unsafe.Pointer(&mtuReq))) // SIOCSIFMTU
+		syscall.Syscall(syscall.SYS_IOCTL, uintptr(s), 0x400c4a52, uintptr(unsafe.Pointer(&mtuReq)))
 	}
 	return ti, nil
 }
@@ -63,6 +63,21 @@ func (ti *tapInterface) Close() error {
 	return syscall.Close(ti.fd)
 }
 
+func (ti *tapInterface) SetMAC(mac [6]byte) error {
+	macStr := fmt.Sprintf("%02x:%02x:%02x:%02x:%02x:%02x", mac[0], mac[1], mac[2], mac[3], mac[4], mac[5])
+	return exec.Command("ip", "link", "set", ti.Name, "address", macStr).Run()
+}
+
+func (ti *tapInterface) SetIP(cidr string) error {
+	if err := exec.Command("ip", "addr", "add", cidr, "dev", ti.Name).Run(); err != nil {
+		return fmt.Errorf("add ip: %w", err)
+	}
+	if err := exec.Command("ip", "link", "set", ti.Name, "up").Run(); err != nil {
+		return fmt.Errorf("link up: %w", err)
+	}
+	return nil
+}
+
 func indexOfZero(s string) int {
 	for i, c := range []byte(s) {
 		if c == 0 {