The spawner of entities of exactly components: on the world's thread, the same function for a world, list and
equal constants each time while an archetype holds each shared value among them, so calling it allocates
nothing; on a joined thread, the same for a list and one constants object, found by its identity. constant gives values every entity it spawns starts with: each shared
component's, which the spawner must be given, since they choose the archetype, and any other's. Its call takes the
rest, or nothing once none of them is required, each component's value from one of the two, never both, which
fails the system; it returns a real entity immediately, alive at the sync point.
Structural changes and writes to other entities, applied in issue order at the next sync point. They take only the components commands may add: every one but those rules derive.