CCCL design: when sequential path (Python forward substitution) dominates,
reduce per-unit work by halving chunk_size from 32→16.
15 loop iterations beats 31, even with 2× more chunks.
Also: add weight-skip warning logs (CCCL ScatterDirect pattern: never
silently discard data). Docker logs will now show exactly which weights
are skipped during load_weights, explaining the 1.12GB gap vs Sub168.
CCCL sources this round:
- block_reduce_warp_reductions.cuh: sequential vs parallel path selection
- warp_exchange_smem.cuh: INSERT_PADDING for memory alignment
- agent_reduce_by_key.cuh: TempStorage union + ScatterDirect