Agent 1 — bulk COLD/WARM migration across init/loader files.
Sites: 39 production HtoD calls migrated to MappedF32Buffer / MappedI32Buffer /
new MappedU32Buffer. Cross-file infrastructure includes new staging helpers
(clone_to_device_{f32,i32}_via_pinned, upload_{f32,i32,u32}_via_pinned),
MappedU32Buffer type, and host_slice_mut accessor on MappedF32/U32Buffer.
Per feedback_no_htod_htoh_only_mapped_pinned.md — first of 3 parallel batches.
# Conflicts:
# docs/dqn-gpu-hot-path-audit.md