Step 3 of β migration: collector now runs aux_next_bar_forward on
rollout state every step. Label producer (new thin variant
aux_sign_label_per_step_kernel) derives sign(price[t+1] - price[t])
per env using bar = episode_starts[ep] + t. Aux predictions feed
the EGF kernel chain (step 4), NOT the Q-head's input (rollout
Q-head still sees raw h_s2, dir_qaux_concat_ptr remains 0u64).
Placement: AFTER captured forward graph, BEFORE expected_q kernel.
Same-stream serial ordering reads exp_h_s2_f32 populated by
forward_online_f32 inside the captured graph. Cold-start gated on
trainer_params_ptr != 0 to skip the test-scaffold path where the
trainer hasn't wired its params yet.
Files added: aux_sign_label_per_step_kernel.cu (66 lines).
Files modified: build.rs (+8 lines, register cubin),
gpu_experience_collector.rs (+106 lines: struct field, cubin static,
load in new(), per-step launch block).
Compile clean; sp14_oracle_tests 2/2 non-GPU pass.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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