[d-v-b]

this PR changes how our big-endian CI job runs, ensuring that we actually get the dependencies we declare, or error.

closes #4546

🤖 ai text below! 🤖
The s390x job installed zarr with --no-deps into an image whose numpy and numcodecs come from Debian sid, so Debian's package versions defined the test environment and zarr's own constraints were never checked. Debian currently ships numcodecs 0.17.0, which zarr excludes (GHSA-jqc8-qjfv-5m6r), and the job failed through the pickle opt-in tests instead of saying so.

Install zarr with its dependencies resolved and run pip check, so an image that does not satisfy pyproject.toml fails at install time. Debian stays a source of prebuilt s390x binaries only. Build a satisfying numcodecs from source in the image, where the compile is cached as a layer, using Debian's meson, Cython and numpy so nothing else compiles under emulation.

Assisted-by: ClaudeCode:claude-fable-5.1

Summary

[Describe what this PR changes and why, in your own words.]

For reviewers

[What would you most value a second look at? What are you already confident in? For a refactor, say whether behavior is meant to be unchanged.]

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)

[d-v-b]

we need this stuff to compile numcodecs, if compilation is necessary (e.g., because debian doesn't ship the version of numcodecs we need)

[d-v-b]

after this change, compiling dependencies for the big-endian CI job can take a long time. since the alternative was to install the wrong dependencies, I think the wait is worth it.

I will be self-merging this when checks clear

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.71%. Comparing base (1dca23f) to head (d0588b2).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4547   +/-   ##
=======================================
  Coverage   94.71%   94.71%           
=======================================
  Files          94       94           
  Lines       13663    13663           
=======================================
  Hits        12941    12941           
  Misses        722      722           

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