Phase E.4.A Task 10 partial: adds the C51 gradient-w.r.t.-input
kernel needed to chain the C51 head's loss gradient back into the
Mamba2 temporal encoder.
Kernel signature: alpha_c51_grad_input_kernel reads probs[B, A, K],
m[B, K], actions[B], W[A*K, in_dim] and writes d_input[B, in_dim].
Math: dL/d_input[b,j] = Σ_k (p[b,a_taken,k] − m[b,k]) · W[a_taken*K+k, j]
(only the taken-action column of W contributes; restricted by the
sparse-over-actions C51 CE gradient structure).
Status: kernel + Rust launcher in place. Mamba2 backward NOT yet
wired in the smoke binary because ml_alpha::Mamba2Block::backward
takes d_logit [B, 1] (post-W_out scalar gradient), not
d_h_enriched [B, hidden_dim]. Two paths to complete:
1. Modify ml-alpha to expose backward_from_h_enriched(cache,
d_h_enriched) bypassing W_out.
2. Replicate the post-W_out backward sequence inline.
Both deferred pending backtest validation (T14): if frozen Mamba2
already lifts backtest Sharpe meaningfully, T10 becomes
optimisation rather than prerequisite. Smoke gate already passed
gate 1 (R_mean +3.9 vs C51-flat -1.1) with frozen weights.
docs/isv-slots.md updated.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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