feat: 网络接口负载采样 — 令牌环认领决策引入真实 NIC 饱和度

- netload_linux.go: 从 /proc/net/dev 字节增量 + /sys/class/net/<iface>/speed
  计算网卡利用率 (rx+tx 合计, 多网卡按容量加权, [0,100] 钳制)
- virtio VM speed=-1 时用 1Gbps 软容量兜底, 保持 VM 上采样有意义
- 排除 lo/docker*/br-*/veth*/tun/tap/wg/virbr* 等非物理 uplink,
  容器桥接流量不再虚增主机负载
- netload_other.go: 非 Linux 平台编译期 fallback 返回 0
- main.go LoadFn: NetPct 从写死的 10 换成 cluster.SampleNetLoad()
- Score = Forwards*100 + MemPct + NetPct: 主信号仍是转发数,
  net 在同转发数节点间打破平局 — NIC 已饱和的节点不再吸引新转发
- 单测: 采样范围断言 + 虚拟接口排除表

实测: 三台集群 net 列显示真实值 (0.02-0.03%), 不再是常量

cmd/webui4frpc/main.go

@@ -135,7 +135,11 @@ func main() {
 		selfID, selfID, *user, *pass, "0.1.0", nil,
 		&cluster.AppHandler{
 			LoadFn: func() (float64, float64) {
-				return cluster.SampleMemLoad(), 10
+				// NetPct: real NIC saturation from /proc/net/dev deltas vs
+				// interface link speed (Linux); 0 fallback elsewhere. A NIC
+				// already near capacity should stop attracting new forwards —
+				// the forward itself is a network hop.
+				return cluster.SampleMemLoad(), cluster.SampleNetLoad()
 			},
 			// Claim persists the forward locally (intermediate config -> store),
 			// then syncs workers so the frpc worker for that remote starts.
@@ -454,7 +458,10 @@ func main() {
 			log.Printf("ring[%s] reconcile: %v", ring.ID, err)
 			return
 		}
-		type ownerKey struct{ local, remote string; port int }
+		type ownerKey struct {
+			local, remote string
+			port          int
+		}
 		ownedBy := map[ownerKey]string{}
 		for _, t := range snap.Topology {
 			k := ownerKey{t.Local.Name, t.Remote.Name, t.Link.RemotePort}