test(drive): cover walkLocalTree's WalkDir error path via a seam

The new root-type guard shifted `filepath.WalkDir`'s outer error branch into
the diff, and neither the macOS nor the Windows runner reaches it naturally —
raising Windows coverage to 99.9365% and blocking the gate. Add a
`statusWalkDir` seam and a `TestCrossPlatformCoverage` regression that swaps
in a WalkDir returning a sentinel error, asserting it is surfaced unchanged.

Verified locally: changed code coverage back to 100.0000%.

internal/helpers/drive_status.go

@@ -470,6 +470,11 @@ var md5File = func(path string) (string, error) {
 // FileInfo 来复现 Windows 上无法创建的符号链接场景。
 var statusRootLstat = os.Lstat
 
+// statusWalkDir 是本地目录遍历的注入 seam。改造根类型校验后，filepath.WalkDir
+// 的错误分支（walkLocalTreeEntry 之外的外层错误）在 macOS/Windows 上都难以自然
+// 触发，需要 seam 让该分支可以确定性回归。
+var statusWalkDir = filepath.WalkDir
+
 // walkLocalTree 递归遍历本地目录，只收集常规文件（跳过符号链接、设备文件等）。
 // rel_path 统一使用 / 作为分隔符，相对于 root。
 //
@@ -491,7 +496,7 @@ func walkLocalTree(root string) (map[string]*localFile, error) {
 		return nil, fmt.Errorf("本地根目录 %q 不是目录", root)
 	}
 	files := make(map[string]*localFile)
-	err = filepath.WalkDir(root, func(path string, d fs.DirEntry, err error) error {
+	err = statusWalkDir(root, func(path string, d fs.DirEntry, err error) error {
 		return walkLocalTreeEntry(root, path, d, err, files)
 	})
 	if err != nil {

internal/helpers/drive_status_coverage_test.go

@@ -5,6 +5,7 @@ import (
 	"encoding/json"
 	"errors"
 	"io"
+	"io/fs"
 	"os"
 	"path/filepath"
 	"runtime"
@@ -325,6 +326,20 @@ func TestCrossPlatformCoverageWalkLocalTree_rejectsSymlinkRootReal(t *testing.T)
 	}
 }
 
+// 根类型校验通过后 WalkDir 内部返回的错误必须原样上抛，不能被吞掉；根消失或
+// 权限变更之类的情形在 Linux/Windows runner 上都难以稳定复现，用 seam 注入。
+func TestCrossPlatformCoverageWalkLocalTree_propagatesWalkDirError(t *testing.T) {
+	testseam.Swap(t, &statusWalkDir, func(string, fs.WalkDirFunc) error {
+		return errWalkDirSentinel
+	})
+	dir := t.TempDir()
+	if _, err := walkLocalTree(dir); !errors.Is(err, errWalkDirSentinel) {
+		t.Fatalf("walk dir error must be surfaced, got %v", err)
+	}
+}
+
+var errWalkDirSentinel = errors.New("simulated walk dir failure")
+
 // 根路径既非目录也非符号链接（例如普通文件被误传）时，也必须报错而非空索引。
 func TestCrossPlatformCoverageWalkLocalTree_rejectsNonDirectoryRoot(t *testing.T) {
 	file := filepath.Join(t.TempDir(), "not-a-dir")