refactor: extract legacy storage interfaces into zarr-storage

[d-v-b]

Summary

This is an LLM-authored PR that copies our store APIs into a subpackage called zarr-storage. This subpackage would be published and versioned separately from zarr. All store APIs from zarr are found in zarr-storage in a legacy module.

This PR contains no changes to zarr. If this PR works, then we would make zarr depend on zarr-storage, and re-export symbols as needed to ensure that no public API changes. A later step would be to develop a new, improved store API in zarr-storage, while preserving the legacy API indefinitely for existing users.

see the original PR, with an AI-authored description, here: d-v-b#325

For reviewers

Happy to bikeshed the layout of zarr_storage.legacy, or explain the motivation for copying these APIs! I started a Zulip thread#Zarr-Python > subpackaging store and codec APIs specifically about this direction, in case folks want a more conversational dialogue.

See also

• fully separate sync and async internal APIs #4217
• how to integrate subpackages #4317
• zarr v3 scale-offset codec, and should the codec API be defined upstream #3867

Author attestation

• I am a human, these are my changes, and I have reviewed and understood every change and can explain why each is correct.

TODO

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/user-guide/*.md
• Changes documented as a new file in changes/
• GitHub Actions have all passed
• Test coverage is 100% (Codecov passes)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.23%. Comparing base (9c29a0d) to head (17a3360).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4318      +/-   ##
==========================================
+ Coverage   94.21%   94.23%   +0.01%     
==========================================
  Files          92       92              
  Lines       12871    12880       +9     
==========================================
+ Hits        12127    12137      +10     
+ Misses        744      743       -1     

see 2 files with indirect coverage changes

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