feat: 新增 IPv6 双栈支持

- 协议: InitMessage 增加 ip6/prefix6/server_ip6 字段(omitempty,向后兼容)
- TUN: Device 接口新增 ConfigureIPv6, macOS 用 ifconfig inet6, Linux 用 ip -6 addr add
- 路由: full 模式自动添加 ::/1+8000::/1, v6 服务器旁路 /128, resolveHosts 双栈解析
- 会话: setupTUN 解析 v6 字段并配置 TUN+路由, connectOnce 传 v6 给 stats/日志
- 统计: SetConnected(ip,ip6), Snapshot.AssignedIP6
- DB: connection_logs 增加 assigned_ip6 列 + migrateV3 幂等迁移
- UI: 状态卡片新增独立 IPv6 行, 始终可见(未获取显示 IPv6: —)
- 文档: 同步 client-development.md 至服务端 IPv6 版本

internal/stats/stats.go

@@ -10,11 +10,11 @@ import (
 type State string
 
 const (
-	StateDisconnected   State = "disconnected"
-	StateConnecting     State = "connecting"
-	StateConnected      State = "connected"
-	StateReconnecting   State = "reconnecting"
-	StateError          State = "error"
+	StateDisconnected State = "disconnected"
+	StateConnecting   State = "connecting"
+	StateConnected    State = "connected"
+	StateReconnecting State = "reconnecting"
+	StateError        State = "error"
 )
 
 // Stats holds live session statistics. Counters are atomic for
@@ -23,8 +23,9 @@ type Stats struct {
 	RxBytes     atomic.Int64
 	TxBytes     atomic.Int64
 	ConnectedAt atomic.Int64 // unix timestamp, 0 = not connected
-	state       atomic.Value  // State
-	assignedIP  atomic.Value  // string
+	state       atomic.Value // State
+	assignedIP  atomic.Value // string (IPv4)
+	assignedIP6 atomic.Value // string (IPv6, may be empty)
 }
 
 // New creates a Stats instance initialised to the disconnected state.
@@ -32,6 +33,7 @@ func New() *Stats {
 	s := &Stats{}
 	s.state.Store(StateDisconnected)
 	s.assignedIP.Store("")
+	s.assignedIP6.Store("")
 	return s
 }
 
@@ -41,10 +43,12 @@ func (s *Stats) SetState(st State) { s.state.Store(st) }
 // State returns the current state.
 func (s *Stats) State() State { return s.state.Load().(State) }
 
-// SetConnected marks the session as connected, recording the time and IP.
-func (s *Stats) SetConnected(ip string) {
+// SetConnected marks the session as connected, recording the time and
+// assigned IP addresses. ip6 may be empty for an IPv4-only server.
+func (s *Stats) SetConnected(ip, ip6 string) {
 	s.ConnectedAt.Store(time.Now().Unix())
 	s.assignedIP.Store(ip)
+	s.assignedIP6.Store(ip6)
 	s.state.Store(StateConnected)
 }
 
@@ -52,18 +56,23 @@ func (s *Stats) SetConnected(ip string) {
 func (s *Stats) SetDisconnected() {
 	s.ConnectedAt.Store(0)
 	s.assignedIP.Store("")
+	s.assignedIP6.Store("")
 	s.state.Store(StateDisconnected)
 }
 
-// AssignedIP returns the server-assigned tunnel IP.
+// AssignedIP returns the server-assigned tunnel IPv4.
 func (s *Stats) AssignedIP() string { return s.assignedIP.Load().(string) }
 
+// AssignedIP6 returns the server-assigned tunnel IPv6 (may be empty).
+func (s *Stats) AssignedIP6() string { return s.assignedIP6.Load().(string) }
+
 // Snapshot returns a point-in-time copy of all counters.
 type Snapshot struct {
 	RxBytes     int64
 	TxBytes     int64
 	ConnectedAt time.Time
 	AssignedIP  string
+	AssignedIP6 string
 	State       State
 	Uptime      time.Duration
 }
@@ -71,10 +80,11 @@ type Snapshot struct {
 // Snapshot returns a point-in-time copy of the statistics.
 func (s *Stats) Snapshot() Snapshot {
 	snap := Snapshot{
-		RxBytes:    s.RxBytes.Load(),
-		TxBytes:    s.TxBytes.Load(),
-		AssignedIP: s.AssignedIP(),
-		State:      s.State(),
+		RxBytes:     s.RxBytes.Load(),
+		TxBytes:     s.TxBytes.Load(),
+		AssignedIP:  s.AssignedIP(),
+		AssignedIP6: s.AssignedIP6(),
+		State:       s.State(),
 	}
 	ts := s.ConnectedAt.Load()
 	if ts > 0 {