Moves MappedF32Buffer + MappedI32Buffer from distributional_q_tests.rs
local definitions to crates/ml/src/cuda_pipeline/mapped_pinned.rs so all
kernel test wrappers (Phase 0.F, upcoming MoE Phase 2 tests) share one
implementation.
Adds write_from_slice helper for direct host_ptr write (no memcpy)
per feedback_no_htod_htoh_only_mapped_pinned.md.
Test 0.F bit-identical post-move: sigma_C51, argmax, Thompson counts
unchanged.
Per spec docs/superpowers/specs/2026-04-27-moe-regime-redesign-design.md §6.4
and feedback_no_htod_htoh_only_mapped_pinned.md.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>