feat(sp22): H6 Phase 3 α — atom-shift wired through compute_expected_q (B9 Step 5+6)

W structural prior init kernel (Step 5):
- aux_w_prior_init_kernel.cu: 4-thread one-time init writing W[a] =
  [-0.5, 0.0, +0.5, 0.0] (Short / Hold / Long / Flat). Hardcoded values
  in kernel — no HtoD per feedback_no_htod_htoh_only_mapped_pinned.md.
- build.rs registration + gpu_dqn_trainer.rs cubin static + handle field
  + new() launch after alloc_zeros for w_aux_to_q_dir.

compute_expected_q atom-shift (Step 6):
- experience_kernels.cu signature grows 4 args (w_aux, batch_states,
  aux_dir_prob_index, state_dim). NULL-safe — collapses to 0 shift
  when either pointer is NULL, bit-identical to pre-Phase-3-α.
- Per-action inner loop computes aux_atom_shift = w_aux[a] * state_121
  once per (b, a) for d==0 only. Inner z-loop applies z_val +=
  aux_atom_shift before all S/TZ/TZ²/TLM accumulators consume it.

Launcher updates (3 trainer sites + 1 collector site):
- populate_q_out: passes W + current states (online path).
- replay_forward_for_q_values: passes W + current states (online replay).
- compute_denoise_target_q: passes W + next_states (target on s'; W
  shared across online/target).
- gpu_experience_collector.rs: passes NULL W + NULL states until
  Phase C1 wires the trainer's W ptr through a setter.

Architectural notes:
- Action selection (every compute_expected_q call) now uses shifted
  atom positions for direction branch (d==0). Other branches stay
  bit-identical (shift = 0). Step 7 (c51_loss_kernel) + Step 8
  (c51_grad_kernel) will close the loop on training loss + W gradient
  in the same atomic commit to avoid the gradient mismatch trap
  per feedback_no_partial_refactor.md.
- Adam wireup for w_aux_to_q_dir lands at Step 11; until then W stays
  at structural prior values (no Adam step modifies it).

Verification:
- cargo check -p ml --lib: 0 errors, 21 pre-existing warnings
  (Phase 3b baseline parity).
- Audit doc updated with B9 Step 5+6 checkpoint entry.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/ml/build.rs

@@ -818,6 +818,15 @@ fn main() {
         // computations integrate into c51_grad_kernel's existing
         // projection backward — no separate α kernel needed. See audit
         // doc Phase 3c entry + spec doc α section for the revised design.
+        //
+        // SP22 H6 Phase 3 α (atom-shift design 2026-05-13): structural-prior
+        // init for w_aux_to_q_dir [4] — writes per-action prior
+        // [-0.5, 0.0, +0.5, 0.0] for [Short, Hold, Long, Flat] at trainer
+        // construction. One-time GPU init (NOT in captured graph). The
+        // Adam-trained W refines from this prior via c51_grad_kernel's
+        // projection backward (Phase 3-final B9 Step 8). Pure 4-thread
+        // single-block; trivial cost.
+        "aux_w_prior_init_kernel.cu",
         // SP14 Layer C Phase C.4b (2026-05-08): adaptive aux prediction
         // horizon producer. Single-thread kernel writing
         // `ISV[AUX_PRED_HORIZON_BARS_INDEX=450]` from