Doubles the trunk capacity. Forward chain:
snap_features → VSN → m1 → LN_a → m2 → LN_b → CfC → GRN heads
m1 = Mamba2Block { in_dim=FEATURE_DIM=40, hidden_dim=128 }
m2 = Mamba2Block { in_dim=128, hidden_dim=128 }
Both stacks share the SAME state_dim (cfg.mamba2_state_dim) and the
SAME hidden_dim. m2 reads m1's output (post LN_a). LN_a is a separate
LayerNorm instance from LN_b (the existing trunk-to-CfC normaliser).
Backward chain reverses the forward:
... grad_ln_in_d → m2.bwd → m2_grads_buffers.d_x_from_in (= LN_a output grad)
→ LN_a.bwd → grad_ln_a_in_d (= m1 output grad)
→ m1.bwd → m1_grads_buffers.d_x_from_in (= VSN output grad)
→ VSN.bwd → ...
Both Mamba2 stacks emit `d_x_from_in` (Phase 2D refactor already
exposed it on m1; m2 uses the same code path). LN_a uses the existing
layer_norm_fwd / layer_norm_bwd / layer_norm_reduce_param_grads
kernels — no new CUDA work, just a second instance with its own
gain/bias/stats/grad scratches.
New trainer state: ~17 fields (mamba2_l2 + its scratch + LN_a + LN_a
grads + opt_ln_a_*). All initialised in the construction order that
respects the `stream` move-into-Self at the end of new().
set_lr_mamba2 now updates BOTH stacks' AdamW configs. Total AdamW
instances on the trainer: 21 (CfC×4 + GRN heads×10 + LN×2 + LN_a×2 +
VSN×2 + Mamba2×2 grouped × 9 params each).
All 8 perception_overfit smokes pass: synthetic constant-direction
signal converges 0.31 → 0.0000 by step 100 (matches single-stack
trajectory — proves both stacks are wired forward + backward and all
21 AdamW optimisers move weights).
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
8f5f22fe4d