The forward_step_bit_identical_after_seed_from_forward_only test in
commit 1d889d2de asserted 1e-5 prediction-level convergence between
forward_only(W[1..K+1]) and seed+forward_step(snap[K]). This is
architecturally impossible: forward_only initialises CfC h_old from a
K-window attention pool; forward_step carries its own hidden state.
Even with a bit-identical SSM seed the two paths see different attention
contexts and diverge in the CfC chain.
The contract seed_step_state_from_forward_only actually makes is at the
BUFFER level: step_scratch_l{1,2}.x_state holds the terminal Mamba2
SSM state produced by replaying K scan_fwd_step calls over the seq
path's pre-computed a_proj/b_proj; and cfc_h_state_step_d is an exact
DtoD copy of h_new_per_k_d[K-1].
Replaced the failing test with seed_step_state_buffers_bit_identical_to_forward_only_terminal:
- Reads cfc_h_state_step_d and h_new_per_k_d[K-1] and asserts bit-for-bit
equality (.to_bits() == .to_bits()) — the DtoD copy makes this exact.
- Verifies L1/L2 x_states are non-zero after seeding (reset zeroed them;
K replay steps built them up).
- Seeds two independent trainers from the same window and asserts all
three buffers match bit-for-bit across both seedings (determinism).
Added readback accessors on the hot path (pub fn, not cfg(test), so
integration tests can reach them — same pattern as forward_step_into_returning):
- Mamba2BlockStepScratch::read_x_state (mamba2_block.rs)
- PerceptionTrainer::read_step_l1_x_state / read_step_l2_x_state /
read_cfc_h_state_step / read_h_new_per_k_last (trainer/perception.rs)
The architectural divergence at prediction level is documented in the
replacement test's docstring so future readers don't reopen the same question.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
3d8f12deba