Advertise tested Python versions

[Fink692]

Summary

• Add Python 3.12 and Python 3.13 classifiers to pyproject.toml.
• Align package metadata with the existing CI matrix and README, which already validate and document Python 3.11–3.13.

This makes the published package metadata accurately communicate the runtimes that the project supports and tests.

Validation

• git diff --check
• pyproject.toml parsed successfully and contains 3.11, 3.12, and 3.13 classifiers
• python3 -m compileall -q src tests
• Existing CI will run Ruff, Black, MyPy, documentation build, dependency audit, and the full 3.11/3.12/3.13 test matrix.