v0.5.0 — citation audit, broader classifiers, Codecov badge

First release with the OSS-promotion polish:

• Citation audit across the repo (src/qqa/__init__.py, both example
  notebooks): the QQA paper (Ichikawa & Arai, ICLR 2025) and the CPRA
  paper (Ichikawa & Iwashita, TMLR 2025) are now cited correctly and
  consistently. Adopted the TMLR-published title for CPRA.
• CITATION.cff preferred-citation block fixed (title and URL now
  match; arXiv:2409.02135 added as an explicit identifier).
• Broadened PyPI classifiers for better discoverability.
• Live Codecov coverage badge added to the README; Zenodo DOI badge
  placeholder ready to activate once this release mints the first DOI.

Full changelog: see CHANGELOG.md.