Adds isToleratedFsyncError, tolerantFsync (async, FileHandle), and
tolerantFsyncSync (sync, fd) helpers that swallow filesystem-limitation
errors during fsync (EPERM, EACCES, ENOTSUP, EINVAL) while still
propagating real errors (EIO, ENOSPC, EBADF, etc.). Synced folders
like iCloud Drive, OneDrive, and antivirus-locked files reject fsync
with EPERM even though the underlying write+rename succeeded; for the
runtime data this codebase persists, losing the durability hint is
acceptable in exchange for not blocking the operation entirely.
The helper is intentionally not barrel-exported (consumers import the
file directly), matching the existing convention for write-file-atomically.
YeonGyu-Kim