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%.
Two follow-ups to the latest CR:
* push/sync uploads (`pushUploadFilePinned`): the PUT-time check pinned inode,
size, and mtime before dispatch but nothing rechecked the source after PUT
succeeded — only the root itself. An editor overwrite, truncate-rewrite, or
mmap-in-place during transfer would land a mixed old/new byte stream in OSS
and still be committed, corrupting the remote file in overwrite/local-wins.
Now stat the still-open handle again before `commit_upload`; any change in
inode/size/mtime aborts the commit. Post-PUT stat failures also abort.
* status root (`walkLocalTree`): `filepath.WalkDir` refuses to follow the root
when it is itself a directory symlink and reports it as a non-regular entry,
so the walker silently returned an empty local index and status flagged
every remote file as `new_remote`. Fail closed before the walk: the root
must be a real directory; symlinks and non-directories are rejected with a
clear message. A `statusRootLstat` seam keeps the rejection regressible on
platforms that cannot create directory symlinks (Windows without admin).
Both fixes come with `TestCrossPlatformCoverage*` regressions and take the
platform coverage gate from 99.9356% back to 100.0000% (1553 statements).
chichuan