Module serial

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Serial-over-chunks SSD with plain autodiff backward.

§SingleSsd Serial (K1–K5) SSD

Chunk-serial counterpart to crate::mamba3::single_ssd::ssd::minimal. Whereas the Minimal variant uses a segsum-based quadratic state passing, this one reuses the K1–K4 helpers from crate::mamba3::double_ssd::ssd::serial (which run a sequential loop for K4) and supplies a new K5 that bakes in the single-ssd logic:

Strict lower-triangular intra-chunk path (the same-time-step block is excluded from the SSM sum; it is the “diagonal correction” territory).
K is scaled by scaleₜ = γₜ + (1−λₜ₊₁) Δₜ₊₁ per source-time column.
Same-time-step block contributes via an explicit γₜ · (C·Bᵀ at t) · Vₜ correction term, restoring the right diagonal weighting.

K1–K4 are identical to the double-SSD because:

K1 (da_cumsum, da_chunk_end) depends only on da = Δ·A.
K2 (cb = C · Bᵀ) is computed on unscaled B / C; the single-ssd algorithm wants the unscaled CB so it can apply scaleₜ per-column (lower triangular) and reuse the same-step block for the γ-correction.
K3 (chunk-end state from V·decay·K) is form-invariant: passing the scale-multiplied K (K_scaled = scaleₜ · B) recovers the single-ssd chunk state, with no other changes needed.
K4 (sequential state passing across chunks) operates on a [H, P, R] per-chunk state and a per-chunk decay total; both are mode-agnostic.

Reference kernels (same as single_ssd_minimal):