Adds the kernel to the build.rs manifest after pearl_1_ext_num_atoms_kernel
and before the SP5 Layer D producers (matching the chronological SP5/SP7
producer ordering). nvcc-compiled cubin lands under
$OUT_DIR/loss_balance_controller_kernel.cubin and is loaded by
gpu_dqn_trainer.rs in T5.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>