All code now uses ptrs.grad_buf (u64) as single source of truth.
Vaccine rewritten to use launch_cublas_backward_to(scratch) instead
of std::mem::swap on CudaSlice which caused pointer divergence
between diagnostics and Adam/graphs. Locally verified: avg_grad=6.13.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>