Make the partial-restore failure test root-proof via a test seam

Why: CI runs as root, where chmod-based failure injection does not
fail (root bypasses file permissions), and any filesystem state that
would fail restore is pre-detected by the classification pass. A
package-level test seam makes the injection deterministic on any uid.
Evidence: GOWORK=off go test (-race) green locally.
Impact: test-only seam, documented as never-for-production.
Refs: Agent-Native#33

runtime/checkpoint/checkpoint.go

@@ -295,12 +295,22 @@ func fileState(group []Record) int {
 
 var errSymlinkTarget = errors.New("checkpoint: restore target is a symlink")
 
+// restoreForTesting 是测试缝：非空时在每次恢复前调用（返回错误即恢复失败）。
+// 生产代码永不应设置它；恢复期的故障注入无法绕开分类层的预检
+// （权限类失败对 root 不可重现，故用显式缝）。
+var restoreForTesting func(abs string) error
+
 func restore(group []Record) error {
 	if len(group) == 0 {
 		return nil
 	}
 	first := group[0]
 	abs := filepath.Join(first.Root, filepath.FromSlash(first.Path))
+	if restoreForTesting != nil {
+		if err := restoreForTesting(abs); err != nil {
+			return err
+		}
+	}
 	// 符号链接防护：恢复会跟出根外（与 fs_write_file 的防护同语义）。
 	// 按外部改动处理——由调用方归入 Skipped，不写不删。
 	if info, err := os.Lstat(abs); err == nil && info.Mode()&os.ModeSymlink != 0 {

runtime/checkpoint/n3_test.go

@@ -1,8 +1,10 @@
 package checkpoint
 
 import (
+	"errors"
 	"os"
 	"path/filepath"
+	"strings"
 	"testing"
 )
 
@@ -78,11 +80,14 @@ func TestRewindPartialFailureContinues(t *testing.T) {
 	if err := log.Record("m1", root, "b/fail.txt", nil, false, []byte("B后")); err != nil {
 		t.Fatal(err)
 	}
-	// dirB 只读：os.Remove 在其中失败。
-	if err := os.Chmod(dirB, 0o500); err != nil {
-		t.Fatal(err)
+	// 测试缝注入恢复失败（权限法对 root 不可重现——CI 以 root 跑）。
+	restoreForTesting = func(abs string) error {
+		if strings.HasSuffix(abs, "b/fail.txt") {
+			return errors.New("注入的恢复失败")
+		}
+		return nil
 	}
-	defer func() { _ = os.Chmod(dirB, 0o755) }()
+	defer func() { restoreForTesting = nil }()
 	res, err := log.Rewind("m1", []string{"m1"})
 	if err != nil {
 		t.Fatalf("单文件失败不得中止：%v", err)