Move all step_synthetic/dqn_replay_step alloc_zeros to persistent
trainer fields (ss_* prefix). Enables CUDA Graph capture of the replay
training step — all device pointers are now stable across steps.
Introduces reduce_axis0_free() to resolve borrow-checker E0502 when
both source (per-batch scratch) and destination (reduced grad) are
self fields passed to the same function.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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