Seven pure-Rust reference tests covering all four training_guard_kernel.cu
kernels: PTX NVRTC smoke-test (skips on CPU-only), NaN/Inf detection,
loss clip, gradient collapse, accumulator averaging, GuardResult boolean
threshold construction, and qvalue_stats_reduce per-sample max-Q reduction.
Zero clippy warnings in the new file.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>