Look up or create the directory handle for bucket. Returns
null if the handle cannot be obtained (e.g., create=false and
the directory does not exist).
Drop the cached handle for bucket. Used after a stale-handle
failure (e.g., "could not be found" from OPFS) so the next access
re-fetches a fresh handle.
Drop every cached handle. Used by OPFSStore.clear() after the
directory tree has been wiped.
Resolve a cache key to its bucket file handle:
<root>/<bucket(key)>/<keyToFileName(key)>.
Cached map of bucket name -> directory handle, with the stale-handle recovery primitive (
invalidate) used after a concurrent clear() leaves dangling handles in memory.