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

internal/cluster/netload_linux_test.go

@@ -0,0 +1,49 @@
+//go:build linux
+
+package cluster
+
+import "testing"
+
+// TestSampleNetLoadBasic: the sampler must stay in [0,100], must return 0 on
+// the first (seeding) call, and must be stable across repeated calls. On a
+// CI/dev box without a speed-known physical NIC it may legitimately return 0
+// every time — that's fine; we only assert the invariants, not a specific
+// value.
+func TestSampleNetLoadBasic(t *testing.T) {
+	first := SampleNetLoad()
+	if first < 0 || first > 100 {
+		t.Fatalf("first sample out of range: %v", first)
+	}
+	for i := 0; i < 3; i++ {
+		v := SampleNetLoad()
+		if v < 0 || v > 100 {
+			t.Fatalf("sample %d out of range: %v", i, v)
+		}
+	}
+}
+
+// TestIsPhysicalUplink: virtual/container interfaces must never count as
+// uplinks — a busy docker bridge would otherwise fake host saturation.
+func TestIsPhysicalUplink(t *testing.T) {
+	cases := []struct {
+		name string
+		typ  int
+		want bool
+	}{
+		{"lo", 772, false},
+		{"ens18", 1, true}, // ARPHRD_ETHER physical
+		{"eth0", 1, true},  // ARPHRD_ETHER physical
+		{"docker0", 1, false},
+		{"br-abc123", 1, false},
+		{"veth9f2c1a", 1, false},
+		{"wg0", 1, false},
+		{"tun0", 65534, false}, // ARPHRD_NONE software iface
+		{"tap7", 1, false},
+		{"bond0", 1, true}, // real aggregated uplink counts
+	}
+	for _, c := range cases {
+		if got := isPhysicalUplink(c.name, c.typ); got != c.want {
+			t.Errorf("isPhysicalUplink(%q,%d)=%v want %v", c.name, c.typ, got, c.want)
+		}
+	}
+}