Per `feedback_no_htod_htoh_only_mapped_pinned.md`, mapped pinned
(cuMemHostAlloc DEVICEMAP) is the only allowed CPU↔GPU path.
`GpuActionSelector::new` previously did one HtoD memcpy to upload
RNG seeds. Now allocates `MappedU32Buffer`, writes seeds via
host_ptr, and passes `dev_ptr` (CUdeviceptr) to the three kernels
that consume rng_states (epsilon_greedy, epsilon_greedy_routed,
branching_action_select).
Adds `MappedU32Buffer` and `MappedU64Buffer` to `mapped_pinned.rs`
mirroring the existing `MappedF32Buffer`/`MappedI32Buffer` API
(new/write_from_slice/read_all/Drop). The U64 variant is staged
for the upcoming spectral-norm host_desc[78] descriptor table
migration in `gpu_dqn_trainer::new`.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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