Function routed_prefill_slot_of 

pub fn routed_prefill_slot_of(
    adapter_slot: i32,
    active: usize,
    num_slots: usize,
) -> Option<usize>

2026-09-25: The predicate behind LoraWeights::routed_prefill_slot. Resolves adapter_slot (>= 0 is that slot, negative is active) and returns it only when it is not active and is < num_slots; otherwise None.