entity::Weak generation is unused

[SOF3]

It is supposed to be cross-checked with generation::WeakStore, but apparently it is not done anywhere.

entity::Weak should not implement entity::Ref directly. It should expose a separate function to use as entity::Ref:

impl<A: Archetype> entity::Weak<A> {
    pub fn try_as_ref(&self, store: impl generation::WeakStore<A>) -> Option<entity::TempRef<'_, A>>;
}