d-v-b's call: PEP 661 is Final, ty already types the sentinel spelling
exactly, mypy support is in review (python/mypy#21647) and treated as
imminent, and pyright has a known-good version — so use the standard
sentinel today rather than carrying the enum stopgap.

- UNSET is now typing_extensions.Sentinel("UNSET"), used directly in
  type expressions (tuple[str | None, ...] | UNSET); the UnsetType
  companion enum is gone from the API.
- typing_extensions floor bumped to 4.14 (where Sentinel arrived).
- CI pins pyright==1.1.404, the last version before the class-attribute
  sentinel regression (microsoft/pyright#11115); pyproject documents the
  same pin for local runs. 0 errors on the pin; ty checks the sentinel
  fields clean (its 2 remaining diagnostics are its incomplete PEP 728
  extra_items write support, unrelated).
- Known short-term cost, accepted deliberately: mypy-checked consumers
  need cast/type-ignore at narrowing sites until mypy#21647 merges, and
  contributors' Pylance may show phantom Unknowns until the pyright fix
  ships. Recorded in _sentinel.py and the changelog.
- The pydantic native-introspection test reverts to documenting that
  introspection is unsupported (pydantic 2.13 cannot schema a Sentinel);
  the delegation patterns are unaffected.

Assisted-by: ClaudeCode:claude-fable-5

The zarr-metadata model layer that zarr now depends on is unreleased, and
the previous wiring only covered uv project flows, breaking everything else:

- The floor 'zarr-metadata>=0.4.0' is unresolvable from PyPI (only <0.4.0
  exists), failing every hatch env (test matrix, doctests, benchmarks,
  hypothesis, min_deps, upstream) and the readthedocs build. The floor now
  names the newest release (>=0.3) with a TODO to bump at release time, and
  each non-uv flow installs the in-tree package explicitly: hatch envs get a
  '{root:uri}/packages/zarr-metadata' direct reference (repeated per env,
  since hatch replaces extra-dependencies on inheritance), readthedocs gets
  an extra pip install step.

- uv workspace membership made 'uv sync' inside packages/zarr-metadata
  resolve the whole workspace, dragging zarr's requires-python (>=3.12) into
  the subpackage's own CI, which tests down to 3.11. The workspace is now a
  plain editable path source, restoring the subpackage's independence.

- min_deps pins typing_extensions==4.16.* (was 4.14.*): 4.16 is the
  effective minimum because zarr-metadata requires it for sentinel pickling.

Also fixes the Lint (mypy) findings in the new tests — a bare generic Array
annotation, stale type-ignores, and two sentinel comparisons mypy cannot
narrow (python/mypy#21647) — and renames the changelog entries to the PR
number (4129) required by the changelog check.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>