Component 5 / Kernel 3 of the SP20 fused-producer chain. Single-block
BLOCK=256 kernel reads `aux_logits [B, 3]` (the SP14-C aux head's
3-class direction logits) and emits `[aux_conf_p50, aux_conf_std]`
into a `MappedF32Buffer<2>`, where the per-row signal is
`aux_conf[i] = max_c softmax(logits[i, *])[c] - 1/3`.
p50 uses the inlined `sp4_histogram_p99` pattern (per-warp tile
binning + cumulative-from-bottom, no atomicAdd per
`feedback_no_atomicadd`); std uses two block tree-reductions sharing
one shmem tile sequentially. One fused kernel streams `aux_logits`
once for both stats per `pearl_fused_per_group_statistics_oracle`.
Phase 1.4 wires the production launch site atomically with the rest
of the SP20 reward chain per `feedback_no_partial_refactor`. This
commit lands kernel + Rust launcher + GPU oracle tests + build entry
+ audit-doc entry together so the kernel is independently verifiable
on RTX 3050 Ti (sm_86) and L40S (sm_89) before the EMA + controller
producers (Phase 1.2 + 1.3) reference its outputs.
Tests verify:
- uniform logits → aux_conf = 0 → [p50, std] = [0, 0]
- varied confidence (logit ramp 0 → 3) → matches CPU oracle
- heterogeneous half-hot half-uniform → matches CPU oracle
- empty batch → degenerate-guard writes [0, 0]
All 4 GPU oracle tests + 4 launcher unit tests pass on RTX 3050 Ti.
Test data uses per-row variance to avoid the
`pearl_sp4_histogram_warp_tile_undercount` lockstep-uniform trap
(concentrated values within one bin_width race the per-warp
non-atomic increments).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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