Make type checking cover tests and match CI

[d-v-b]

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Splits the type-checking work out of #180 so the justfile PR can build on it.

The problem

hatch run types:check failed immediately with 227 errors, so just typecheck in #180 would have been broken out of the box. Two independent causes:

• The types environment installed only mypy, pydantic and numpy — no zarr, pytest, xarray or dask. With ignore_missing_imports = true those all became Any, producing ~90 phantom errors (untyped-decorator on every pytest mark, stale unused-ignore comments).
• The mypy that actually gated CI was the mirrors-mypy pre-commit hook, pinned to v1.15.0 and scoped to files: src. The hatch env resolved a different mypy and checked tests too, so the two could never agree.

Provisioning the environment correctly dropped this to 134 errors and made src/ clean — the 19 apparent source errors were also artifacts of the missing zarr install.

Single entry point

hatch run types:check is now the only type-check command, run locally and by a new typecheck CI job:

• [tool.hatch.envs.types] gains dependency-groups = ["test"] and pins mypy==2.3.0, so the versions mypy checks against come from the same place the tests get them and cannot drift.
• The mypy pre-commit hook is removed. sp-repo-review's PC140 hard-requires that hook, so it joins the existing [tool.repo-review] ignore list.

Trade-off worth naming: there is no longer mypy feedback at git commit time; the CI job is what catches it.

Real bugs found

1. from_zarr overloads declared depth without a default in both v2 and experimental.v2, so the documented one-argument call from_zarr(group) did not type-check — for downstream users too, not just this repo. v3 was already correct.
2. experimental.v2.GroupSpec.from_flat was narrower than to_flat: to_flat() returns BaseGroupSpec values but from_flat() accepted only GroupSpec, so GroupSpec.from_flat(group.to_flat()) did not type-check. Widened to match the module-level from_flat, which already accepted it.
3. A test assertion that could only pass as None == None: spec.filters is a tuple while auto_filters() returns the raw list, so any non-empty case would have failed. Now normalised before comparing.

The remainder is mechanical: ~40 stale type: ignore comments removed, missing annotations added, and isinstance narrowing in place of # type: ignore[attr-defined] on .members chains — which makes those assertions stronger rather than merely quieter.

Verification

Check	Result
hatch run types:check (src + tests)	Success, 21 files
pre-commit run --all-files	all hooks pass
tests, zarr 3.0.10 / 3.1.0 / 3.3.0	19568 / 19690 / 19690 passed
tests, no-zarr env	17876 passed
doctests	30 passed

Only src change with runtime effect is none — the two source fixes are annotation-level. tests/conftest.py does change at runtime (Structured(fields=...) now takes a tuple), which is why all three zarr versions were exercised.

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