Replace all 20 dangling `ptx` variable references with correct cubin
static names after nvrtc removal sed. Fix ml-dqn dead code stubs
(residual.rs, rmsnorm.rs, noisy_layers.rs, gpu_replay_buffer.rs).
Clean up unused `let ptx` variables.
Zero compilation errors across full workspace.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>