feat: Refine dy.Categorical to support pl.Categories

[Daniel Elsner (delsner)]

Motivation

Polars allows setting name, namespace and physical backing type for categoricals. Right now, dataframely users cannot specify these parameters, even though they're relevant for efficient categorical representation.

Changes

Add support for pl.Categories. I decided to create our own data structure since pl.Categories isn't serializable out of the box.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (febf6ce) to head (6267c85).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #378   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           56        56           
  Lines         3507      3543   +36     
=========================================
+ Hits          3507      3543   +36     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[Copilot]

Pull request overview

This PR extends dy.Categorical so callers can specify Polars-style global category metadata (name/namespace and physical index type) via a new serializable dy.Categories helper, enabling more explicit and potentially more efficient categorical representations.

Changes:

• Introduces dy.Categories and adds a categories parameter to dy.Categorical, mapping to pl.Categories when constructing the Polars dtype.
• Extends column serialization/deserialization to roundtrip the new categorical metadata.
• Adds unit tests covering dtype construction, equality/matching, and schema serialization roundtrips for categorical columns.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
tests/schema/test_serialization.py	Adds categorical schemas to the general schema serialization roundtrip parametrization.
tests/column_types/test_categorical.py	New tests for categorical dtype mapping, matching, dict roundtrip, and schema serialization roundtrip.
dataframely/columns/categorical.py	Adds Categories dataclass, pl.Categories dtype support, and serialization hooks for categorical metadata.
dataframely/columns/init.py	Re-exports Categories from the columns package.
dataframely/init.py	Exposes Categories at the top-level dataframely API.