Vigour is spent, never restored
Pairing two esvas consumes vigour, and vigour does not come back. Each parent carries six uses. When the sixth is spent, that specimen never breeds again — it stays playable and stays sellable, and its record reads "Vigour 0/6" from then on.
What the process costs and gives
Incubation takes eight hours. You start with one slot and can hold up to two extra. The offspring inherits three IVs, or six with a lock. A deviation can appear in breeding; the chance is very low, and it stays that way.
Why the rule is permanent
An irreversible cost is the only thing separating a lineage from a factory. Without it, every strong parent becomes an assembly line and rarity dies within weeks. The limit is not a friction tax — it is what keeps a bred esva worth something to the person who bred it.