ReplaceWindows owns the unresolved-import decision: a POSIX call that does not
resolve (DllNotFound/EntryPointNotFound) is "unsupported"; a MoveFileExW that
does not resolve degrades to the portable File.Replace/Move. Tests for each
exception type, the last-attempt same-attempt fallback and the retried-87 case.
Doc and Mesh test comments no longer claim a reader never finds the path absent;
the Mesh race test tolerates a brief not-found on Windows.
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Both store writers call StoreReplace.Replace; the directory store's File.Replace
retry loop and WriteSnapshotAtomically's Exists/Replace/Move block are deleted.
Un-ignores the mesh empty-store race test on Windows.
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>