Add Cramer's V contingency metric

[Kkkakania]

Description

Add Cramér's V as a multi-category metric on Contingency. The implementation uses xarray operations throughout, preserves Dataset dimensions and Dask laziness, and excludes empty rows or columns when determining the effective table size.

Closes #75

Type of change

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

• compared a 2x2 result with scipy.stats.contingency.association(method="cramer")
• covered Dataset and Dask inputs, empty declared categories, and invalid one-category tables
• uv run --extra test pytest -q (2798 passed, 3 skipped)
• pre-commit on all four changed files, including Ruff, blackdoc, doc8, and mypy

Checklist (while developing)

• I have added a docstring and API reference entry for the new method.
• The implementation is expressed through named xarray dimensions and operations.
• Tests added for pytest.

Pre-Merge Checklist (final steps)

• Based on the current upstream main.
• Contains one focused commit.

References

• Cramér's V definition: https://en.wikipedia.org/wiki/Cram%C3%A9r%27s_V
• SciPy reference implementation: https://docs.scipy.org/doc/scipy/reference/generated/scipy.stats.contingency.association.html

AI assistance

I used an AI coding assistant to help inspect the existing contingency API and draft tests. I reviewed the statistical implementation, compared it with SciPy, and ran the complete test and pre-commit suites locally.

[Kkkakania]

I investigated the failed Read the Docs build. It reaches the existing tabular-data.ipynb gallery notebook and fails while fetch_california_housing() looks for a missing local cal_housing.pkz dataset file. That notebook and its data-loading path are unrelated to the new contingency metric. The focused contingency tests, full local suite, typing, and pre-commit checks for this PR pass.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.65%. Comparing base (e457985) to head (fa6dd72).
⚠️ Report is 11 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #459      +/-   ##
==========================================
+ Coverage   94.57%   94.65%   +0.07%     
==========================================
  Files          27       27              
  Lines        2841     2920      +79     
==========================================
+ Hits         2687     2764      +77     
- Misses        154      156       +2     

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[aaronspring]

Couldn’t this work within apply_ufunc using xr min()