Closes#57
- Add RARITY_LADDER and rarity_is_downgrade() to rarity_thresholds.py
- Add get_fully_evolved_players() to db_calls.py — queries a to-be-created
database endpoint; returns empty set safely if endpoint is unavailable
- In batters/creation.py post_player_updates(): pre-flight check identifies
players where OPS rarity would downgrade, then guards the rarity write to
skip any downgrade for fully-evolved (T4) cards
- Same guard added to pitchers/creation.py post_player_updates()
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>