packaging: add Python 3.13 classifier

[CoderDeltaLAN]

Summary

• adds the official Python 3.13 package classifier
• aligns package metadata with the existing Python 3.13 compatibility CI job
• keeps requires-python unchanged at >=3.12

Research basis

• PyPI supports the Programming Language :: Python :: 3.13 classifier
• PyPA packaging docs treat classifiers as package metadata/search metadata
• requires-python remains the installation compatibility gate

Checks

• ./scripts/check.sh
• ./scripts/post-release-audit.sh

Boundaries

No code, workflow, release, tag, PyPI, dependency, or branch-protection changes.