fix: ensureDaemon 自动检测并重启过期守护进程

问题: 重新构建后 IPv6 等新功能不生效,客户端仍运行旧代码。
根因: ensureDaemon() 通过 IPC socket 拨号时,若旧 daemon 进程仍在
运行(重建后未退出),GUI 会直接复用旧进程,永不启动新二进制。
日志证实旧 daemon 处理连接(init 日志无 ip6 字段),新代码从未执行。

解决方案: 新增版本握手协议,GUI 启动时查询 daemon 构建版本,
版本不匹配则自动关闭旧进程并启动新的。

- 新建 internal/version 包,GUI 与 daemon 共享同一 Version 变量,
  均通过 Makefile ldflags 注入
- IPC 新增 CmdVersion 命令,daemon 返回 version.Version;
  Response 增加 Version 字段,Client 新增 QueryVersion() 方法
- ensureDaemon() 拨号后查询版本,不匹配则发 CmdShutdown、等待
  socket 释放、重新启动 daemon;旧 daemon 不识别 CmdVersion 时
  返回错误,同样触发重启
- Makefile LDFLAGS 改为注入 lmvpn/internal/version.Version

internal/ipc/ipc.go

@@ -29,6 +29,7 @@ const (
 	CmdStop     = "stop"
 	CmdShutdown = "shutdown"
 	CmdStats    = "stats"
+	CmdVersion  = "version" // query daemon build version
 )
 
 // Event types sent from daemon to GUI.
@@ -49,8 +50,8 @@ type Request struct {
 // package (which needs root-only TUN) into the GUI.
 type ClientConfig struct {
 	ServerURL   string   `json:"server_url"`
-	SNIHost     string   `json:"sni_host"`    // TLS SNI hostname for CDN
-	ServerIPs   []string `json:"server_ips"`  // CDN edge IP list for failover
+	SNIHost     string   `json:"sni_host"`   // TLS SNI hostname for CDN
+	ServerIPs   []string `json:"server_ips"` // CDN edge IP list for failover
 	Username    string   `json:"username"`
 	Password    string   `json:"password"`
 	Token       string   `json:"token"`
@@ -203,8 +204,9 @@ func (c *Client) Close() error { return c.conn.Close() }
 
 // Response is a simple ack/error reply to a command.
 type Response struct {
-	OK    bool   `json:"ok"`
-	Error string `json:"error,omitempty"`
+	OK      bool   `json:"ok"`
+	Error   string `json:"error,omitempty"`
+	Version string `json:"version,omitempty"` // set for CmdVersion replies
 }
 
 // WriteOK sends a success response to a connection.
@@ -232,6 +234,28 @@ func SendShutdown(c *Client) error {
 	return c.Send(Request{Cmd: CmdShutdown})
 }
 
+// WriteVersion sends a version response to a connection.
+func WriteVersion(conn net.Conn, ver string) error {
+	return writeMsg(conn, Response{OK: true, Version: ver})
+}
+
+// QueryVersion sends a CmdVersion request and reads the daemon's
+// build version. It must be called before any VPN session starts (so
+// no Event messages are in flight on the connection).
+func (c *Client) QueryVersion() (string, error) {
+	if err := c.Send(Request{Cmd: CmdVersion}); err != nil {
+		return "", err
+	}
+	var resp Response
+	if err := readMsg(c.r, &resp); err != nil {
+		return "", err
+	}
+	if !resp.OK {
+		return "", fmt.Errorf("version query failed: %s", resp.Error)
+	}
+	return resp.Version, nil
+}
+
 // RoutingModeFromIPC converts an IPC routing mode string to route.Mode.
 func RoutingModeFromIPC(s string) route.Mode {
 	switch s {