Drop Python 3.13 and obsolete PyTorch dependencies

[adamtheturtle]

Summary

• require Python 3.14 and update CI, lint, documentation, and site publishing to use it
• update vws-python-mock to the current PyTorch-free release
• remove the explicit torch and torchvision development dependencies, CPU-only package index, and deptry exceptions
• use a type-checking-only Generator import now that Python 3.14 defers annotation evaluation
• add a release-note fragment for the Python support change

Verification

• uv run --extra=dev pytest -q --cov-fail-under=100 --cov=src/ --cov=tests . (16 passed, 100% coverage)
• all prek stages (pre-commit, pre-push, and manual) over all files