Per `feedback_no_htod_htoh_only_mapped_pinned.md`, mapped pinned
(cuMemHostAlloc DEVICEMAP) is the only allowed CPU↔GPU path.
Migrates 3 WARM HtoD sites:
- `fused_training.rs:505` HER source_indices Vec<i32> upload
- `fused_training.rs:511` HER reward_ones Vec<f32> upload
- `training_loop.rs:470` curriculum episode_starts Vec<i32> upload
For `fused_training.rs` adds module-level `staging_upload_{i32,f32}`
helpers. For `training_loop.rs` uses an inline staging+DtoD block
because the destination is reached via the out-of-scope accessor
`collector.episode_starts_buf_mut() -> &mut CudaSlice<i32>`.
All destination buffer types remain `CudaSlice<T>` so no downstream
consumer (incl. `gpu_her::relabel_batch_with_strategy` and any
`episode_starts_buf` reader) needs to change.
Note: if Agent 2's `gpu_experience_collector` merge changes
`episode_starts_buf_mut`'s return type to a mapped pinned buffer,
the inline staging block in `training_loop.rs:470` can be simplified
to a direct `write_from_slice` (follow-up).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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