docs: release notes for 0.17.0

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Prepares the release notes for 0.17.0. No code changes — docs/release.md only.

Why 0.17.0 and not 0.16.6

Per the versioning policy in docs/contributing.md, micro releases are for "no changes to the public API", which doesn't hold here. Strictly read, that policy calls for a major bump on backwards-incompatible changes, but the project has never behaved that way — #622 dropped Python 3.10 / NumPy 1.23 in 0.14.0 under Maintenance, and 0.13.1 shipped a documented breaking change in a patch. De facto, breaking changes land in minors while we're on 0.x.

The Python/NumPy drop alone would be weak grounds by that precedent. What makes this a minor is the numcodecs.tests removal, the build-system migration, and the free-threading work in #858.

What changed in the notes

• Added missing entries. Neither Drop Python 3.11 and NumPy <2.0 per SPEC-0 #834 (Python 3.12 / NumPy 2.0 minimums) nor update pcodec #817 (pcodec 1.x) was documented at all.
• Recategorized into Breaking changes. The tests relocation (Move tests out of installable distribution #829) and the removal of DISABLE_NUMCODECS_CEXT (Migrate build system from setuptools to meson-python #826) were under Maintenance — the latter buried mid-paragraph in the meson entry. The two numcodecs.blosc behavioral changes from Add free-threaded CPython (3.14t) support #858 (removal of get_mutex, and the sem_open fallback change) moved out of Enhancements, with the free-threading support itself staying there and the two entries cross-referencing each other.
• Clarified the blast radius of Move tests out of installable distribution #829. Tests are gone from the installed wheel, but tests/ and fixture/ are still in the sdist — confirmed by building one locally with mesonpy.build_sdist (all 40 test files present, and pyproject.toml carries testpaths = ["tests"]). So distro packagers are unaffected; the break only hits code doing import numcodecs.tests.common at runtime. Worth stating explicitly, since "tests were removed" reads much scarier than it is.
• Documented the delta_spec deprecation from update pcodec #817 ("none" → "no_op", still works, now warns).
• Fixed leftover RST roles. The existing Unreleased entries used :user: and :issue:, which don't render under MyST — collateral from the RST→Markdown conversion in Convert RST docs to markdown #830. Also converted an RST :: literal block to a fenced code block.
• Added entries for the remaining PRs since v0.16.5, including CI: set an explicit python-version for test-crc32c and test-zarr #856 and Bump GitHub actions #838.

Follows the convention from #804: the new version section goes below an empty Unreleased heading.

Note for a follow-up

.github/labeler.yml still matches on !docs/release.rst, but #830 renamed that file to docs/release.md. Since release.rst can no longer match, the needs release notes label now fires on every PR unconditionally. Not fixed here to keep this PR to release notes — happy to send a one-liner separately.

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