Removes run_nan_tracing (3 stream syncs per step on first 20 steps)
and unused grad_buf_ptr/params_buf_ptr accessors. Root causes are
fixed — runtime detection no longer needed. isfinite guards in CUDA
kernels remain as silent defense-in-depth.
17/17 smoke tests pass.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>