Function resolve_pending_metas 

pub fn resolve_pending_metas(
    metas: &mut [Meta],
    classify: &mut dyn FnMut(u32, bool) -> SlotContribution,
)

Resolve every Meta::Pending in metas in dependency order (MetaRef edges form a forest — each meta is referenced by at most one parent walk; explicit stack, so deep ?rule chains cannot recurse out). classify reads a drained slot’s host value (peek, never consume) and MUST be callable for every slot a plan references — the binding calls this only when drains guarantee those slots are filled (a crossing’s consuming drain; parse finish). Unresolvable plans (opaque residuals) demote to Meta::Poisoned, which surfaces as Empty.