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/tracker/tracker.go

@@ -7,24 +7,39 @@ import (
 	"os"
 	"os/exec"
 	"path/filepath"
+	"sort"
+	"strings"
 	"sync"
+	"time"
 )
 
 type Tracker struct {
-	mu         sync.Mutex
-	dataDir    string
-	workDir    string
-	lowerDir   string
-	upperDir   string
-	mergeDir   string
-	mounted    bool
-	active     bool
-	before     *FSState
-	changeSets []*ChangeSet
+	mu              sync.Mutex
+	dataDir         string
+	workDir         string
+	lowerDir        string
+	upperDir        string
+	mergeDir        string
+	mounted         bool
+	active          bool
+	before          *FSState
+	changeSets      []*ChangeSet
+	keepChangesets  int     // 保留最近 N 份 changeset，0 = 不限
+	maxChangesetAge time.Duration // changeset 最大保留时长，0 = 不限
 }
 
-func NewTracker(dataDir, workDir string) *Tracker {
-	return &Tracker{
+type TrackerOption func(*Tracker)
+
+func WithKeepChangesets(n int) TrackerOption {
+	return func(t *Tracker) { t.keepChangesets = n }
+}
+
+func WithMaxChangesetAge(d time.Duration) TrackerOption {
+	return func(t *Tracker) { t.maxChangesetAge = d }
+}
+
+func NewTracker(dataDir, workDir string, opts ...TrackerOption) *Tracker {
+	t := &Tracker{
 		dataDir:    dataDir,
 		workDir:    workDir,
 		lowerDir:   filepath.Join(workDir, "lower"),
@@ -32,6 +47,10 @@ func NewTracker(dataDir, workDir string) *Tracker {
 		mergeDir:   filepath.Join(workDir, "merged"),
 		changeSets: make([]*ChangeSet, 0),
 	}
+	for _, opt := range opts {
+		opt(t)
+	}
+	return t
 }
 
 func (t *Tracker) Init() error {
@@ -40,6 +59,7 @@ func (t *Tracker) Init() error {
 			return fmt.Errorf("create overlay dir %s: %w", d, err)
 		}
 	}
+	t.cleanupChangeSets()
 	log.Printf("[tracker] initialized (work=%s)", t.workDir)
 	return nil
 }
@@ -172,6 +192,65 @@ func (t *Tracker) saveChangeSet(cs *ChangeSet) {
 	}
 }
 
+func (t *Tracker) cleanupChangeSets() {
+	dir := filepath.Join(t.dataDir, "changesets")
+	entries, err := os.ReadDir(dir)
+	if err != nil {
+		return
+	}
+
+	type csFile struct {
+		name  string
+		info  os.FileInfo
+	}
+	var files []csFile
+	for _, e := range entries {
+		if e.IsDir() || !strings.HasSuffix(e.Name(), ".json") {
+			continue
+		}
+		info, err := e.Info()
+		if err != nil {
+			continue
+		}
+		files = append(files, csFile{name: e.Name(), info: info})
+	}
+
+	// 按修改时间排序
+	sort.Slice(files, func(i, j int) bool {
+		return files[i].info.ModTime().Before(files[j].info.ModTime())
+	})
+
+	now := time.Now()
+	remaining := make([]csFile, 0, len(files))
+
+	for _, f := range files {
+		keep := true
+
+		if t.maxChangesetAge > 0 && now.Sub(f.info.ModTime()) > t.maxChangesetAge {
+			keep = false
+		}
+
+		if keep {
+			remaining = append(remaining, f)
+		}
+	}
+
+	// 再按数量裁剪
+	if t.keepChangesets > 0 && len(remaining) > t.keepChangesets {
+		excess := len(remaining) - t.keepChangesets
+		for i := 0; i < excess; i++ {
+			path := filepath.Join(dir, remaining[i].name)
+			os.Remove(path)
+		}
+		remaining = remaining[excess:]
+	}
+
+	if len(files) != len(remaining) {
+		log.Printf("[tracker] cleanup: removed %d changesets, kept %d",
+			len(files)-len(remaining), len(remaining))
+	}
+}
+
 func (t *Tracker) Rollback() error {
 	t.mu.Lock()
 	defer t.mu.Unlock()
@@ -213,11 +292,13 @@ func (t *Tracker) Stats() map[string]interface{} {
 		totalChanges += len(cs.Files)
 	}
 	return map[string]interface{}{
-		"mounted":         t.mounted,
-		"active":          t.active,
-		"change_sets":     len(t.changeSets),
-		"total_changes":   totalChanges,
-		"merge_dir":       t.mergeDir,
-		"upper_dir":       t.upperDir,
+		"mounted":           t.mounted,
+		"active":            t.active,
+		"change_sets":       len(t.changeSets),
+		"total_changes":     totalChanges,
+		"merge_dir":         t.mergeDir,
+		"upper_dir":         t.upperDir,
+		"keep_changesets":   t.keepChangesets,
+		"max_changeset_age": t.maxChangesetAge.String(),
 	}
 }