Monster Evolution

Monsters transforming into stronger species at level thresholds — the genre's signature long-term reward.

Why deferred: Evolution needs target species with their own sprites, i.e. new art, plus an evolution chain in the species data and a reveal sequence. All mechanical prerequisites (level-ups, species data map, monster instances keyed by MonsterKey) already exist, so this is blocked on content, not architecture.

Revisit when: new species art exists (the same trigger that unblocks roster growth beyond five).