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%.
chichuan
0c2a9cb2b3