mlx-bun library API
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    interface ExpertResidencyManagerOptions {
        backend: ExpertResidencyBackend;
        locate: (layer: number, expertId: number) => ExpertResidencyLocation;
        pinned?: readonly { expertId: number; layer: number }[];
        plan: ExpertResidencyPlan;
        routeObserver?: (
            layer: number,
            routes: readonly ExpertUsageRoute[],
        ) => void;
        sparseLayerIds: readonly number[];
        usage?: ExpertUsageLedger;
    }
    Index
    locate: (layer: number, expertId: number) => ExpertResidencyLocation
    pinned?: readonly { expertId: number; layer: number }[]
    routeObserver?: (layer: number, routes: readonly ExpertUsageRoute[]) => void

    Synchronous measurement seam; receives router rows before union/dedup.

    sparseLayerIds: readonly number[]