Atomic flip of the aux heads' input from Q's GRN trunk output `save_h_s2`
to the SEPARATE aux trunk's output `h_s2_aux`. The aux trunk now trains
its own w1/w2/w3/b1/b2/b3 from CE loss (next-bar + regime); Q's encoder
is structurally protected by `aux_trunk_backward`'s missing `dx_in`
output param (encoder boundary stop-grad enforced at the kernel-set
level).
Reverts the C.0 stop-grad band-aid commits (`872bd7392`, `411a30473`):
the zero-fills in `aux_next_bar_backward` + `aux_regime_backward` Step 3
are replaced with the genuine SAXPY-back-to-input gradient
(`dh_s2_aux[b,j] = sum_k sh_dh_pre[k] * w1[k,j]`). The leak that
motivated stop-grad is now blocked structurally rather than by data
zero-fill — aux gradient flows through the aux trunk's own params, never
into Q's encoder.
Wired in this commit (atomic, ~330 LOC):
- 4 kernel signatures renamed `h_s2 → h_s2_aux` / `dh_s2_out → dh_s2_aux_out`
(`aux_next_bar_forward`, `aux_regime_forward`, `aux_next_bar_backward`,
`aux_regime_backward`); Rust wrappers in `gpu_aux_heads.rs` follow
- Trainer fwd: insert `aux_trunk_forward_ops.launch(...)` in
`aux_heads_forward` Step 0, populating `h_s2_aux` from `save_h_s1`
(encoder layer-1 output, dim=shared_h1=256). Both head fwds redirect
input pointer from `save_h_s2` to `h_s2_aux`
- Trainer bwd: SAXPY both `aux_dh_s2_*_buf` into `dh_s2_aux_accum`
(pre-zeroed each step via graph-safe `cuMemsetD32Async`); then
`aux_trunk_backward_ops.launch(...)` propagates through w3/w2/w1 +
b3/b2/b1; then `launch_aux_trunk_adam_update` applies global L2-norm
clip + per-tensor Adam updates over 6 grad tensors
- Collector fwd: insert `exp_aux_trunk_forward_ops.launch(...)` after
`forward_online_f32`, reading `exp_h_s1_f32` and writing `exp_h_s2_aux`;
redirect `exp_aux_heads_fwd.forward_next_bar` input from
`exp_h_s2_f32` to `exp_h_s2_aux`
- Pre-capture host-write of ISV-driven LR + grad-clip + step counter
into mapped-pinned buffers in `launch_cublas_backward_to` (BEFORE
`aux_heads_backward`); same `&mut self` pattern as `step_ofi_embed_adam`
Verification:
- `cargo check -p ml --tests` clean (1m02s, only pre-existing warnings)
- `aux_trunk_oracle_tests` + `sp14_oracle_tests` 12/12 pass:
- aux_trunk gradient check: max_rel_err=1.33e-2 (tol=2e-2) — matches C.4 baseline
- aux_trunk_backward_does_not_write_dx: kernel source clean of dx_in/dx_in_out
- aux_sign_label_lookahead_mask: 60/100 masked, 40/100 valid
- 9 other oracle tests pass bit-identically
Plan: docs/superpowers/plans/2026-05-07-sp14-layer-c-separate-aux-trunk.md §C.5b
Audit: docs/dqn-wire-up-audit.md "SP14 Layer C Phase C.5b" section
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