ppf bug

[alexanderthclark]

Summary

• Fix PPF addition when multiple component PPFs have identical slopes.
• Sort PPF components by slope using a key function instead of sorting (slope, curve) tuples, avoiding comparisons between curve objects when slopes tie.
• Add regression coverage for adding two and three same-slope PPFs.
• Refresh the branch against current main after PR Add Bach or Stravinsky helper #85 merged.
• Leave the package version unchanged; version bumps should be handled separately for releases.

Testing

• pytest -q tests/test_curves.py -k "PPF or BaseQuadratic" -> 10 passed