nx_ssm_gate.nx

nx_ssm_gate.nx -- GATE for SSM-001: the CAUSAL state-space mixer (the autoregressive sub-quadratic lane, vs FNet's bidirectional encoder lane). Proves, by RUNNING: G1 GRADCHECK through the autograd: x[4,2] + learnable decay a[2] -> ta_ssm -> mse(.,target); analytic gradients for BOTH the input dx AND the decay da (backprop-through-time) vs central finite difference (h=1/128, rel<1/32 floor 1/64). Proves the reverse-scan backward is correct. G2 CAUSALITY: perturbing the input at the LAST time step leaves every EARLIER output unchanged, and changes only the last output -- i.e. the operator's Jacobian is lower-triangular (y_t sees only x_<=t). This is what FNet cannot do and what autoregressive generation requires. Evidence -> knowledge/status/ssm.log (SSMGATE authored=organ ... verdict=GREEN). license_tier: ORIGINAL