Stacked on top of feat: Added RFC-9 Zipped OME-Zarr (PR #408).

Two ``pack_ozx`` runs over the same OME-Zarr source previously
produced different ``.ozx`` bytes — and therefore different sha256s
— because Python's ``zipfile`` module stamps every Local File Header
with ``time.localtime()[:6]`` when called as ``zout.open(arcname,
mode="w")``. Two more variable fields piggyback on that path:
``create_system`` (3 on Unix, 0 on Windows) and ``external_attr``
(file-mode bits sniffed from the OS).

Non-determinism here is a problem for any artifact-integrity
workflow downstream — the ML Commons Croissant 1.1 spec embeds
``cr:FileObject.sha256`` per archive, and per-dataset MANIFEST.json
files (e.g. dynacell-paper's distribution layer) record the sha256
that gets published to AWS Open Data. Without byte reproducibility,
the verification chain breaks the moment someone re-packs a source
zarr that has not actually changed.

Fix: build a ``ZipInfo`` per entry with every variable field pinned,
then pass it to ``zout.open(zinfo, mode="w", force_zip64=True)``
instead of the bare arcname. New helper ``_reproducible_zip_info``
sets:

- ``date_time = (1980, 1, 1, 0, 0, 0)`` — the zip epoch, earliest
  representable timestamp.
- ``create_system = 3`` — Unix, regardless of the host OS that ran
  the pack.
- ``external_attr = (S_IFREG | 0o644) << 16`` — regular-file 0o644
  in the upper 16 bits where Unix-creator entries store mode bits.
- ``compress_type = ZIP_STORED`` — already enforced at the ZipFile
  level; per-entry pin is belt-and-braces.
- ``force_zip64=True`` on every ``zout.open`` call so the ZIP64
  decision is uniform; otherwise zipfile picks per-entry based on
  size, which can flip for borderline-sized chunks.

Existing functional behaviour is unchanged — the round-trip,
HCS-layout, and version-sniffing tests all still pass. RFC-9
compliance also unaffected: BFS entry order, archive comment, and
ZIP_STORED compression are all preserved.

Tests:
- ``test_pack_ozx_is_byte_reproducible``: pack the same source
  twice, assert ``a.read_bytes() == b.read_bytes()``. Fails on the
  pre-fix branch.
- ``test_pack_ozx_pinned_zip_metadata``: walk every entry's
  ``ZipInfo`` and assert ``date_time``, ``create_system``, and
  ``external_attr`` carry the pinned values. Catches a future
  refactor that drops one of the pins (which would still pass the
  byte-equality test for two same-machine same-second runs but
  silently break cross-machine reproducibility).

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>