mlx-bun library API
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    ServedKv:
        | { kind: "bf16" }
        | { bits: number; groupSize: number; kind: "affine" }
        | { kind: "affine-layers"; layers: readonly AffineKvLayer[] }
        | { kBits: number; kind: "turbo"; vBits: number }

    A KV scheme as a table entry serves it: its storage, the kind and widths. A composition's scheme also names the affine read kernels (kernels) and the delayed conversion start (quantizedKvStart); the cache composes both from the composition, so neither constrains the graph.