GpuBatch now stores raw u64 device pointers to pre-allocated replay
buffer memory. Eliminates per-step:
- 14 cuMemAlloc calls (7 in sample_proportional + 7 in into_gpu_batch)
- 14 DtoD copies (clone into owned CudaSlice)
- CPU staging Vec and flush() dead code path
Also removed: GpuBatchSlices, into_gpu_batch, dtod_clone_* helpers,
StagedGpuBuffer.staging field, CPU add/add_batch for GpuPrioritized.
update_priorities_cuda now takes u64 raw pointer.
HER relabel_batch_with_strategy takes u64 episode_ids_ptr.
Cold-path Q-value estimation uses compute_q_stats_internal.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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