feat(backup): 新增差异备份（differential）模式 (#88)

文件备份新增差异模式：仅打包自上次全量以来的变更并记录删除，恢复自动按全量+差异链还原。含基线解析、链式恢复、保留链保护与本机文件任务校验；清单/比对/删除/往返/保留保护单测全覆盖。

server/internal/backup/manifest_test.go

@@ -0,0 +1,79 @@
+package backup
+
+import (
+	"reflect"
+	"testing"
+)
+
+func TestEncodeDecodeManifestRoundTrip(t *testing.T) {
+	m := Manifest{Entries: []ManifestEntry{
+		{Path: "src/a.txt", Size: 10, ModTimeNs: 100, Mode: 0o644},
+		{Path: "src", Size: 0, ModTimeNs: 50, Mode: 0o755, IsDir: true},
+	}}
+	data, err := EncodeManifest(m)
+	if err != nil {
+		t.Fatalf("EncodeManifest: %v", err)
+	}
+	got, err := DecodeManifest(data)
+	if err != nil {
+		t.Fatalf("DecodeManifest: %v", err)
+	}
+	if !reflect.DeepEqual(got, m) {
+		t.Fatalf("roundtrip mismatch:\n got %#v\nwant %#v", got, m)
+	}
+}
+
+func TestDecodeManifestEmpty(t *testing.T) {
+	got, err := DecodeManifest(nil)
+	if err != nil {
+		t.Fatalf("DecodeManifest(nil): %v", err)
+	}
+	if len(got.Entries) != 0 {
+		t.Fatalf("expected empty manifest, got %#v", got)
+	}
+}
+
+func TestChangedSince(t *testing.T) {
+	base := Manifest{Entries: []ManifestEntry{
+		{Path: "a.txt", Size: 10, ModTimeNs: 100},
+		{Path: "dir", IsDir: true, ModTimeNs: 100},
+	}}.index()
+
+	cases := []struct {
+		name string
+		cur  ManifestEntry
+		want bool
+	}{
+		{"unchanged file", ManifestEntry{Path: "a.txt", Size: 10, ModTimeNs: 100}, false},
+		{"size changed", ManifestEntry{Path: "a.txt", Size: 11, ModTimeNs: 100}, true},
+		{"mtime changed", ManifestEntry{Path: "a.txt", Size: 10, ModTimeNs: 200}, true},
+		{"new file", ManifestEntry{Path: "b.txt", Size: 1, ModTimeNs: 1}, true},
+		{"existing dir skipped", ManifestEntry{Path: "dir", IsDir: true, ModTimeNs: 999}, false},
+		{"new dir included", ManifestEntry{Path: "newdir", IsDir: true, ModTimeNs: 1}, true},
+	}
+	for _, tc := range cases {
+		if got := changedSince(base, tc.cur); got != tc.want {
+			t.Errorf("%s: changedSince=%v want %v", tc.name, got, tc.want)
+		}
+	}
+}
+
+func TestDeletedPaths(t *testing.T) {
+	base := Manifest{Entries: []ManifestEntry{
+		{Path: "a"}, {Path: "b"}, {Path: "c"},
+	}}.index()
+	seen := map[string]struct{}{"a": {}, "c": {}}
+	got := deletedPaths(base, seen)
+	want := []string{"b"}
+	if !reflect.DeepEqual(got, want) {
+		t.Fatalf("deletedPaths=%v want %v", got, want)
+	}
+}
+
+func TestDeletedPathsNoneWhenAllSeen(t *testing.T) {
+	base := Manifest{Entries: []ManifestEntry{{Path: "a"}, {Path: "b"}}}.index()
+	seen := map[string]struct{}{"a": {}, "b": {}}
+	if got := deletedPaths(base, seen); len(got) != 0 {
+		t.Fatalf("expected no deletions, got %v", got)
+	}
+}