Module schedule 

Gate builders for the system table (define_systems! in registry). Each gate inspects the world’s content and returns the constructed system when its gating components are present, or None to leave it out of the schedule. World::start and World::system_manifest both run these same gates, so what the manifest reports and what start builds cannot drift.

Gates construct their system, so every system constructor must stay cheap and side-effect-free: the manifest probe discards the value, and anything heavy (device acquisition, payload reads) belongs in System::init.