[d-v-b]

prs that don't touch subpackages are triggering subpackage docs builds, which are failing. this PR fixes the two problems therein.

1. it updates our trigger for subpackage docs builds by comparing changes against the merge base instead of main. this ensures that only PRs that touch subpackages trigger subpackage docs builds in CI.
2. it pins pymdown-extensions to a version compatible with mkdocstrings 1.0.6. this keeps the subpackage docs builds from failing.

Here's claude's rendition of that same thing:

🤖 AI text below 🤖

The zarr-indexing, zarr-metadata and zarr-http-server docs sites install their docs group with pip on Read the Docs, without a lockfile, so pymdown-extensions floated to 12.3, whose Highlight extension API the pinned mkdocstrings 1.0.6 cannot drive (Highlight.init() missing argument 'md'). No zarr-indexing build has succeeded since. Pin pymdown-extensions to 11.0.1, the version the repo-root uv.lock resolves to, and refresh the package lockfiles.

The post_checkout skip filters compared the PR head against the tip of main with a two-dot diff, so any PR that was behind main appeared to touch every package and built every site. Compare against the merge base instead, after fetching full history into the shallow checkout. Also exclude zarr-http-server from the root site's filter, as the other two packages already were.

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]

I will self-merge this when checks are green.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.71%. Comparing base (90e72a9) to head (9b21193).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4543   +/-   ##
=======================================
  Coverage   94.71%   94.71%           
=======================================
  Files          94       94           
  Lines       13663    13663           
=======================================
  Hits        12941    12941           
  Misses        722      722           

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