docs: 3.4.0 release notes - #4360

AI-authored release notes PR

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Summary

Release notes for zarr 3.4.0, generated with uv run --only-group release towncrier build --version 3.4.0 on main at 4484a60b (77 commits past v3.3.0, including the release blog post in #4355). The 33 changelog fragments are consumed; the new section has 3 features, 22 bugfixes, 4 documentation entries and a misc line.

Two touch-ups to the generated text: the codec error-message example inherited a code fence with no language from its fragment, which markdownlint rejects, so it is now a text fence; and the three fragments that had been named after the issue they resolve rather than the pull request that merged them (3285 from #4063; 4174 and 4272 both from #4218) now link to those pull requests, per the changelog convention that fragments carry PR numbers. Everything else is verbatim from the fragments.

The strict docs build passes with the new section, and the pre-commit hooks (markdownlint, codespell) pass on the file.

Also folded in: a four-line formatting fix to the 3.4.0 blog post. The "Why are we doing this?" bullets had each label on its own line with the explanation on the next, which Markdown folds into a run-on ("Modularity Installing zarr brings in…"); the labels are now bold lead-ins on the same line.

This is the "Open a release PR with the changelog entries" step of #4256. Remaining after merge: tag v3.4.0 targeting main, verify PyPI and Read the Docs, and the conda-forge feedstock PR — which needs a manual recipe edit for the new msgspec>=0.19 dependency.

For reviewers

The content is the fragments as merged; review is mostly for the ordering and the two edits above. The version is 3.4.0 rather than a 3.3.x patch because main carries three feature entries (msgspec-based metadata validation with a new required dependency, the codec error changes, and numpy-array chunk specs in create_array). SPEC 0's numpy floor (2.2) was left alone for this release, per the checklist discussion.

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

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