feat(sp22-vnext): Phase A4 — aux_trade_outcome loss reduce kernel
K=3 sparse cross-entropy reduce over the trade-outcome softmax tile
produced by `aux_trade_outcome_forward` (Phase A3). Mirrors the K=2
sibling `aux_next_bar_loss_reduce` structurally: single-block shmem-tree
reduce, two parallel partial strips (loss_numer + valid_count) reduced
lockstep, fmaxf(p_tgt, 1e-30) numerical floor, fmaxf(valid, 1.0) all-
skip-batch guard, valid_count_out[1] save-for-backward.
Kept as SEPARATE kernel from the K=2 sibling:
- Diagnostic isolation (distinct HEALTH_DIAG slot, distinct cubin in
profiles for clean per-loss-source attribution)
- Sparse-label semantic clarity (~95-99% mask=-1 vs ~50-100% valid for
the K=2 next-bar head)
- Future per-class weighting headroom (Profit/Stop/Timeout 3:1-10:1
imbalance will likely need class-weighted CE — surgical mod here
without touching the K=2 head's contract)
Phase A4 (this commit) is dead code — no Rust launcher yet. Phase A5
lands backward; Phase B wires the full forward→loss→backward chain.
Discipline: feedback_no_atomicadd (single-block tree-reduce), feedback_
cpu_is_read_only (pure GPU), pearl_first_observation_bootstrap (sentinel
0 valid_count produces zero gradients gracefully on cold start).
Audit: docs/dqn-wire-up-audit.md Phase A4 section.
Cubin: aux_trade_outcome_loss_reduce_kernel.cubin (9.9 KB) compiles clean.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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