fix: 修复 Windows 端 IPv6 无法下行的问题

根因: netsh interface ipv6 命令参数名用错
- configureIPv6Addr: name= → interface=,IPv6 地址未实际分配到适配器
- SetMTU IPv6: name= → interface=

路由修复:
- cidrToV6Prefix: 用 Mask.Size() 取前缀长度,修复 fmt.Sprintf(%d, mask) 输出字节切片的 bug
- addRoute6/deleteRoute6: netsh → route -6,用 :: 作 on-link gateway
- addRouteVia6/deleteRouteVia6: 加 -6 flag
- defaultGateway6: route -6 print :: → route -6 print(避免过滤掉 ::/0)
- applyFull: IPv6 bypass 路由失败不再阻断全隧道路由

internal/tun/tun_windows.go

@@ -116,12 +116,16 @@ func (d *wintunDevice) ConfigureIPv6(localIP6 net.IP, prefix6 int) error {
 func (d *wintunDevice) configureIPv6Addr(ip net.IP, prefix int) error {
 	addr := fmt.Sprintf("%s/%d", ip.String(), prefix)
 	return execCmd("netsh", "interface", "ipv6", "add", "address",
-		"name="+d.name, "address="+addr)
+		"interface="+d.name, "address="+addr)
 }
 
 func (d *wintunDevice) SetMTU(mtu int) error {
-	return execCmd("netsh", "interface", "ipv4", "set", "subinterface",
-		"name="+d.name, fmt.Sprintf("mtu=%d", mtu))
+	if err := execCmd("netsh", "interface", "ipv4", "set", "subinterface",
+		"name="+d.name, fmt.Sprintf("mtu=%d", mtu)); err != nil {
+		return err
+	}
+	return execCmd("netsh", "interface", "ipv6", "set", "subinterface",
+		"interface="+d.name, fmt.Sprintf("mtu=%d", mtu))
 }
 
 // ensureWintunDLL extracts the embedded wintun.dll to the executable's