Add polars/narwhals support to the formula interface

[Martin Stancsics (stanmart)]

This PR follows up on #370 and #388 by adding generic dataframs support for the formula interface.

Checklist

• Added a CHANGELOG.rst entry

[Marc-Antoine Schmidt (MarcAntoineSchmidtQC)]

Look's good. I tested the speed and nw.from_native is always extremely fast, although faster for polars. But this is not going to be relevant in any of our benchmarks.

[Copilot]

Pull request overview

This PR adds support for polars and other dataframes supported by narwhals to the formula interface, building on previous work. It updates the Python requirement from 3.9 to 3.10 and upgrades the formulaic dependency from 0.6 to 1.2.

Changes:

• Integrated narwhals for generic dataframe support in TabmatMaterializer and encode_contrasts
• Refactored _InteractableCategoricalVector.from_categorical to from_codes for framework-agnostic categorical handling
• Added comprehensive test parametrization to run all formula tests with both pandas and polars dataframes

Reviewed changes

Copilot reviewed 9 out of 10 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
tests/test_formula.py	Parametrized all formula tests to run with both pandas and polars; updated fixtures to support multiple input types; updated categorical vector creation to use new from_codes API
src/tabmat/formula.py	Added narwhals integration to TabmatMaterializer; refactored categorical handling to use codes/categories instead of pandas-specific objects; updated _C and encode_contrasts functions for generic dataframe support
src/tabmat/categorical_matrix.py	Added support for polars Enum types in _extract_codes_and_categories_polars
src/tabmat/benchmark/generate_matrices.py	Added type ignore comment for type checking
setup.py	Removed Python 3.9 support, updated formulaic requirement to >=1.2, bumped minimum Python to 3.10
pyproject.toml	Updated mypy python_version from 3.9 to 3.10
pixi.toml	Updated Python requirements, formulaic version, polars version, removed py39 environment
conda.recipe/meta.yaml	Updated formulaic requirement to >=1.2
CHANGELOG.rst	Documented new narwhals/polars support and Python 3.10 requirement

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