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

pkg/types/types.go

@@ -46,10 +46,12 @@ type Heartbeat struct {
 }
 
 type NetworkCheckResult struct {
-	LLMAPIReachable   bool   `json:"llm_api_reachable"`
-	DNSResolving      bool   `json:"dns_resolving"`
+	LLMAPIReachable   bool          `json:"llm_api_reachable"`
+	DNSResolving      bool          `json:"dns_resolving"`
+	TCPReachable      bool          `json:"tcp_reachable"`
 	Latency           time.Duration `json:"latency_ms"`
-	Error             string `json:"error,omitempty"`
+	LatencyDegraded   bool          `json:"latency_degraded"`
+	Error             string        `json:"error,omitempty"`
 }
 
 type SnapshotPolicy struct {