plan(sp18 v2): Phase 0 Task 0.1 — D-leg per-action reward decomposition diagnostic

Adds the SP18 v2 Phase 0 D-leg observability scaffold per the plan's
"Phase 0 — diagnostic emit (NO functional change)" task:

- New kernel `reward_decomp_diag_kernel.cu`: block tree-reduce
  (4 blocks × 256 threads, one block per direction-axis bin) reading
  `reward_components_per_sample [N×6]` + `actions_out [N]` and emitting
  5 per-bin stats (mean r_micro / mean r_opp_cost / mean r_popart /
  mean |reward| / fire_rate) into a 20-float row-major output. Bin
  order: Hold(0)→Long(1)→Short(2)→Flat(3); col order: micro→opp→
  popart→abs→fire. Empty-bin guard emits 0.0 (NOT NaN) per the
  consumer-side KILL CRITERION arithmetic contract.

- Cubin manifest entry in `crates/ml/build.rs` + `REWARD_DECOMP_DIAG_
  CUBIN` re-export in `gpu_dqn_trainer.rs`.

- 20-float `MappedF32Buffer sp18_reward_decomp_diag_buf` field on
  `GpuDqnTrainer` + accessor pair (`sp18_reward_decomp_diag_dev_ptr`
  for the writer-side launcher; `read_sp18_reward_decomp_diag` for the
  HEALTH_DIAG reader). Buffer is constructor-zeroed so cold-start
  HEALTH_DIAG emits a deterministic zero block.

- `sp18_reward_decomp_diag_kernel` field on `GpuExperienceCollector` +
  cubin load on the collector's stream + `launch_sp18_reward_decomp_
  diag(n, b1, b2, b3, out_dev_ptr)` launcher. Wired in
  `training_loop.rs` at the per-step boundary, BEFORE
  `launch_reward_component_ema_inplace` (which `memset_zeros` the
  source buffer after consuming it) per `pearl_canary_input_freshness_
  launch_order`.

- New per-epoch HEALTH_DIAG line emit at the existing per-epoch
  boundary (after the SP17 dueling line):

    HEALTH_DIAG[N]: reward_decomp [hold(micro=X opp=Y popart=Z abs=W
                    fire=F) long(...) short(...) flat(...)]

  Reads the mapped-pinned 20-float diag buffer directly via the
  collector→trainer host_ptr — no DtoH copy.

- New `crates/ml/tests/sp18_hold_reward_oracle_tests.rs`:
  * `reward_decomp_per_action_cpu_oracle` (CPU oracle pinning the
    per-bin reduction math against a 4-sample synthetic batch).
  * `reward_decomp_per_action_gpu_oracle` (GPU oracle, ignored unless
    `--ignored`; asserts kernel matches CPU oracle bit-for-bit within
    1e-6 f32 budget).
  * `reward_decomp_empty_bin_emits_zero_not_nan` (empty-bin contract
    guard).

Pure observability — no production-path consumer in this commit. No
reward changes, no Bellman target changes, no kernel modifications to
the action-selection or training paths. Per
`feedback_no_partial_refactor` the kernel + cubin manifest + buffer +
launcher + production wire-up + HEALTH_DIAG emit + GPU oracle test all
land atomically.

Verification:
  SQLX_OFFLINE=true CUDA_COMPUTE_CAP=86 cargo check --workspace clean.
  CPU oracle test passes; GPU oracle + empty-bin guard both pass on
  RTX 3050 Ti (2.13s).

Plan: docs/superpowers/plans/2026-05-08-sp18-reward-shape-hold-attractor.md
      § Phase 0 Task 0.1.
Audit: docs/dqn-wire-up-audit.md § "SP18 v2 Phase 0 Task 0.1".

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

crates/ml/build.rs

@@ -879,6 +879,21 @@ fn main() {
         // `isv[ISV_HOLD_COST_IDX=380]` by `isv[461]`. See
         // docs/dqn-wire-up-audit.md § "SP16 Phase 2" for rationale.
         "hold_cost_scale_update_kernel.cu",
+        // SP18 v2 Phase 0 Task 0.1 (2026-05-08): D-leg per-action reward
+        // decomposition diagnostic. Reads `reward_components_per_sample`
+        // ([n_samples × 6]) + `actions` ([n_samples], factored-action
+        // ints) from the experience collector, bins per direction
+        // (DIR_HOLD / DIR_LONG / DIR_SHORT / DIR_FLAT — 4 bins), and
+        // emits 5 stats per bin (mean r_micro, mean r_opp_cost, mean
+        // r_popart, mean |reward|, fire_rate) into a 20-float
+        // mapped-pinned `reward_decomp_diag_buf`. Block tree-reduce
+        // (4 blocks × 256 threads, one block per bin); no atomicAdd
+        // per `feedback_no_atomicadd.md`. Pure observability — does
+        // NOT modify any production-path buffer; written values are
+        // consumed exclusively by the per-epoch HEALTH_DIAG line emit.
+        // Plan: docs/superpowers/plans/2026-05-08-sp18-reward-shape-hold-attractor.md
+        // § Phase 0 Task 0.1.
+        "reward_decomp_diag_kernel.cu",
         // SP14 Layer C Phase C.6 (2026-05-08): h_s2_aux RMS EMA producer.
         // Single-block 256-thread kernel computing RMS = sqrt(mean(x²))
         // over `h_s2_aux [B, SH2]` (aux trunk final output, no activation)