- GpuBufferPool: reuses pre-allocated staging buffers for zero-alloc fold transitions
instead of always calling DqnGpuData::upload() directly
- DoubleBufferedLoader: new field on DQNTrainer for zero-downtime fold transitions;
skips re-upload when active slot is already populated from a previous fold
- Added double_buffer() / double_buffer_mut() accessors
- Upload path: check double-buffer first, then try buffer_pool, then direct upload
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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