Macro define_component_storage 

macro_rules! define_component_storage {
    (
        storage: $storage:ident,
        slot: $slot:ident,
        $( $field:ident => $ty:path, $disc:expr ),+ $(,)?
    ) => { ... };
}

Generate a component storage for a fixed set of component types: one Column<T> per type, the entity allocator, the shared change tick, the join index, and the slot access trait that resolves a type to its column at compile time.

The expanding site names the concrete component types; this module stays engine-agnostic.