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/file_runner_diff_test.go

@@ -0,0 +1,141 @@
+package backup
+
+import (
+	"archive/tar"
+	"context"
+	"io"
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func diffWrite(t *testing.T, p, content string) {
+	t.Helper()
+	if err := os.MkdirAll(filepath.Dir(p), 0o755); err != nil {
+		t.Fatalf("mkdir %s: %v", filepath.Dir(p), err)
+	}
+	if err := os.WriteFile(p, []byte(content), 0o644); err != nil {
+		t.Fatalf("write %s: %v", p, err)
+	}
+}
+
+func diffAssertContent(t *testing.T, p, want string) {
+	t.Helper()
+	got, err := os.ReadFile(p)
+	if err != nil {
+		t.Fatalf("read %s: %v", p, err)
+	}
+	if string(got) != want {
+		t.Fatalf("%s content = %q, want %q", p, string(got), want)
+	}
+}
+
+func diffAssertAbsent(t *testing.T, p string) {
+	t.Helper()
+	if _, err := os.Stat(p); !os.IsNotExist(err) {
+		t.Fatalf("expected %s to be absent, stat err=%v", p, err)
+	}
+}
+
+func diffArchiveNames(t *testing.T, artifactPath string) map[string]bool {
+	t.Helper()
+	f, err := os.Open(artifactPath)
+	if err != nil {
+		t.Fatalf("open artifact: %v", err)
+	}
+	defer f.Close()
+	names := map[string]bool{}
+	tr := tar.NewReader(f)
+	for {
+		h, err := tr.Next()
+		if err == io.EOF {
+			break
+		}
+		if err != nil {
+			t.Fatalf("read tar: %v", err)
+		}
+		names[h.Name] = true
+	}
+	return names
+}
+
+// TestFileRunnerDifferentialRoundTrip 验证差异备份的端到端正确性：
+// 全量 → 修改源（变更/删除/新增）→ 差异 → 链式恢复（全量+差异）→ 结果与修改后源一致。
+func TestFileRunnerDifferentialRoundTrip(t *testing.T) {
+	work := t.TempDir()
+	src := filepath.Join(work, "src")
+	diffWrite(t, filepath.Join(src, "a.txt"), "alpha")
+	diffWrite(t, filepath.Join(src, "b.txt"), "bravo")
+	diffWrite(t, filepath.Join(src, "sub", "c.txt"), "charlie")
+
+	runner := NewFileRunner()
+
+	full, err := runner.Run(context.Background(), TaskSpec{Name: "diff", Type: "file", SourcePath: src, TempDir: t.TempDir()}, NopLogWriter{})
+	if err != nil {
+		t.Fatalf("full Run: %v", err)
+	}
+	if full.Manifest == nil || len(full.Manifest.Entries) == 0 {
+		t.Fatalf("full backup must produce a manifest, got %#v", full.Manifest)
+	}
+
+	// 变更 a.txt（内容变长 → size 差异必被检出）、删除 b.txt、新增 d.txt；sub/c.txt 不变
+	diffWrite(t, filepath.Join(src, "a.txt"), "ALPHA-modified-and-longer")
+	if err := os.Remove(filepath.Join(src, "b.txt")); err != nil {
+		t.Fatalf("remove b.txt: %v", err)
+	}
+	diffWrite(t, filepath.Join(src, "d.txt"), "delta")
+
+	diff, err := runner.Run(context.Background(), TaskSpec{Name: "diff", Type: "file", SourcePath: src, TempDir: t.TempDir(), Differential: true, BaseManifest: *full.Manifest}, NopLogWriter{})
+	if err != nil {
+		t.Fatalf("differential Run: %v", err)
+	}
+	if diff.Manifest != nil {
+		t.Fatalf("differential backup must not produce a manifest")
+	}
+
+	// 差异归档应包含变更/新增条目与删除清单，但不含未变更的 sub/c.txt
+	names := diffArchiveNames(t, diff.ArtifactPath)
+	if !names["src/a.txt"] || !names["src/d.txt"] {
+		t.Fatalf("differential archive missing changed/new entries: %v", names)
+	}
+	if names["src/sub/c.txt"] {
+		t.Fatalf("differential archive should not contain unchanged file sub/c.txt")
+	}
+	if !names[deletionsEntryName] {
+		t.Fatalf("differential archive missing deletions entry: %v", names)
+	}
+
+	// 链式恢复到全新目标
+	restoreRoot := t.TempDir()
+	restoreSrc := filepath.Join(restoreRoot, "src")
+	restoreTask := TaskSpec{Name: "diff", Type: "file", SourcePath: restoreSrc}
+	if err := runner.Restore(context.Background(), restoreTask, full.ArtifactPath, NopLogWriter{}); err != nil {
+		t.Fatalf("restore full: %v", err)
+	}
+	if err := runner.Restore(context.Background(), restoreTask, diff.ArtifactPath, NopLogWriter{}); err != nil {
+		t.Fatalf("restore differential: %v", err)
+	}
+
+	diffAssertContent(t, filepath.Join(restoreSrc, "a.txt"), "ALPHA-modified-and-longer")
+	diffAssertContent(t, filepath.Join(restoreSrc, "sub", "c.txt"), "charlie")
+	diffAssertContent(t, filepath.Join(restoreSrc, "d.txt"), "delta")
+	diffAssertAbsent(t, filepath.Join(restoreSrc, "b.txt"))
+}
+
+// TestFileRunnerDifferentialWithoutBaseIsFull 验证无基线时差异请求回退为全量（产出清单、含全部文件）。
+func TestFileRunnerDifferentialWithoutBaseIsFull(t *testing.T) {
+	src := filepath.Join(t.TempDir(), "src")
+	diffWrite(t, filepath.Join(src, "a.txt"), "alpha")
+
+	runner := NewFileRunner()
+	res, err := runner.Run(context.Background(), TaskSpec{Name: "diff", Type: "file", SourcePath: src, TempDir: t.TempDir(), Differential: true}, NopLogWriter{})
+	if err != nil {
+		t.Fatalf("Run: %v", err)
+	}
+	if res.Manifest == nil {
+		t.Fatalf("differential without base must fall back to full and produce a manifest")
+	}
+	if names := diffArchiveNames(t, res.ArtifactPath); !names["src/a.txt"] || names[deletionsEntryName] {
+		t.Fatalf("fallback-full archive unexpected: %v", names)
+	}
+}