fix: reset the codec-pipeline thread pool after fork

[barlowa124]

Summary

Fixes #4478. zarr.core.codec_pipeline keeps a module-level ThreadPoolExecutor for FusedCodecPipeline. A forked child inherits _pool pointing at threads that do not exist in the child, plus the parent's idle-semaphore permits. submit consumes a permit and returns without spawning a thread, so submissions up to the permit count queue work that nothing runs. Small multi-chunk reads hang forever. Larger reads recover once they exhaust the stale permits and spawn a real thread.

reset_resources_after_fork in zarr.core.sync resets the event loop and IO thread plus the sync executor after a fork, but the codec pipeline registered no after_in_child hook. This adds one that drops the inherited _pool and _pool_size and recreates _pool_lock, since the parent may have held it at fork time, which would leave the child's copy permanently locked.

The regression test reads a multi-chunk array in the parent to populate the pool, then reads a 2-chunk region in a forked child. Two chunks is below the inherited permit count, so unfixed code hangs and the test fails on a 30s get timeout instead of blocking CI.

For reviewers

Two things I would value a second look at are the decision to recreate _pool_lock rather than reuse the inherited lock, and whether any other module-level executor state needs the same treatment. Locally the issue's ProcessPoolExecutor script timed out at 30s unfixed and passed with the fix. The permit arithmetic was confirmed by printing _idle_semaphore._value in parent and child.

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 94.71%. Comparing base (ab4f2fe) to head (8d225da).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4483   +/-   ##
=======================================
  Coverage   94.71%   94.71%           
=======================================
  Files          94       94           
  Lines       13659    13663    +4     
=======================================
+ Hits        12937    12941    +4     
  Misses        722      722           

Files with missing lines	Coverage Δ
src/zarr/core/codec_pipeline.py	95.58% <100.00%> (+0.03%)	⬆️

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[d-v-b]

looks good, thank you!