Function gpu_pool_for_policy 

pub fn gpu_pool_for_policy(
    policy: EncodePolicy,
    codec: VideoCodec,
) -> Arc<GpuPool>

Build a GpuPool constrained to the given EncodePolicy. An empty pool (e.g. a pinned index or vendor family that isn’t present) yields capacity 0, so the orchestrator’s pre-flight probe / lease claim surfaces a clear error.

When more than one GPU is selected, cards that can’t actually encode the REQUESTED codec (e.g. a pre-Ada NVIDIA that decodes via NVDEC but has no AV1 encode silicon — yet can still encode H.264/H.265) are dropped from the encode pool, so a worker never leases an incapable card and hard-fails the run; the capable cards do the encoding. A single selected GPU is left as-is, since the serial path’s non-pinned encoder dispatch already falls through vendors. Dropped cards stay available for the decode pump (policy_gpu_indices is intentionally NOT filtered).