Add an io::fsync module with a per-OS split (posix/windows) mirroring the
writeback_file convention, replacing two duplicated dir-fsync copies:
- dir(): POSIX directory fsync; a no-op on Windows, where std cannot open
a directory as a File and the failed open logged a spurious warning on
every mapfile write.
- file_durable(): opens the target read+write before sync_all so the flush
succeeds on Windows, where FlushFileBuffers rejects a read-only handle
with ERROR_ACCESS_DENIED.
Point the mapfile writer at the shared dir() helper.
