Root cause of H100 gradient collapse: c51_grad, mse_grad, and cql_grad
kernels wrote d_adv_logits in sample-major interleaved layout [B, TBA]
but cuBLAS backward read it as branch-major [B*B0*NA | B*B1*NA | ...].
At batch=16384, the GEMM read cross-contaminated data from wrong branches,
producing zero weight gradients by symmetry cancellation at scale.
Fix: changed indexing in all 3 grad kernels to branch-major.
Reverted diagnostic hacks (ungraphed forward, forced diagnostics).
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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