fix: 网络检测增强 + Tracker changeset 清理

网络检测 (monitor.go):
- TCP 拨测 (8.8.8.8:53 / 1.1.1.1:53 / 208.67.222.222:53)
- 延迟阈值检测 (>5s 标记降级)
- DNS 多目标检测 (google/baidu/cloudflare)
- NetworkCheckResult 新增 TCPReachable + LatencyDegraded

Tracker:
- 新增 WithKeepChangesets / WithMaxChangesetAge 选项
- Init 时自动清理过期 changeset (默认保留100份/30天)
- 先按年龄裁剪，再按数量裁剪

internal/network/monitor.go

@@ -12,11 +12,11 @@ import (
 )
 
 type Monitor struct {
-	mu       sync.RWMutex
-	client   *http.Client
-	interval time.Duration
+	mu        sync.RWMutex
+	client    *http.Client
+	interval  time.Duration
 	endpoints []string
-	status   []EndpointStatus
+	status    []EndpointStatus
 }
 
 type EndpointStatus struct {
@@ -142,7 +142,11 @@ func (m *Monitor) AggregateResult() types.NetworkCheckResult {
 	m.mu.RLock()
 	defer m.mu.RUnlock()
 
-	result := types.NetworkCheckResult{LLMAPIReachable: true, DNSResolving: true}
+	result := types.NetworkCheckResult{
+		LLMAPIReachable: true,
+		DNSResolving:    true,
+		TCPReachable:    true,
+	}
 	var totalLatency time.Duration
 	checked := 0
 
@@ -161,14 +165,49 @@ func (m *Monitor) AggregateResult() types.NetworkCheckResult {
 		result.Latency = totalLatency / time.Duration(checked)
 	}
 
-	result.DNSResolving = m.checkDNS()
+	// 延迟阈值检测：平均延迟 > 5s 标记为降级
+	if result.Latency > 5*time.Second {
+		result.LatencyDegraded = true
+		if result.Error == "" {
+			result.Error = fmt.Sprintf("high latency: %v", result.Latency)
+		}
+	}
+
+	// DNS 多目标检测
+	result.DNSResolving = m.checkDNSMulti()
+
+	// TCP 拨测：检测基础网络通畅性
+	result.TCPReachable = m.checkTCPReachability()
+
 	return result
 }
 
-func (m *Monitor) checkDNS() bool {
-	_, err := net.LookupHost("google.com")
-	if err != nil {
-		_, err = net.LookupHost("baidu.com")
+func (m *Monitor) checkDNSMulti() bool {
+	targets := []string{"google.com", "baidu.com", "cloudflare.com"}
+	for _, target := range targets {
+		_, err := net.LookupHost(target)
+		if err == nil {
+			return true
+		}
 	}
-	return err == nil
+	return false
+}
+
+func (m *Monitor) checkTCPReachability() bool {
+	targets := []struct {
+		host string
+		port string
+	}{
+		{"8.8.8.8", "53"},
+		{"1.1.1.1", "53"},
+		{"208.67.222.222", "53"},
+	}
+	for _, t := range targets {
+		conn, err := net.DialTimeout("tcp", net.JoinHostPort(t.host, t.port), 3*time.Second)
+		if err == nil {
+			conn.Close()
+			return true
+		}
+	}
+	return false
 }