feat(sp5): Task A5 — Pearl 5 per-branch IQN τ schedule GPU producer (Layer A)
Two new CUDA kernels land as SP5 Layer A additive producers feeding ISV[250..270)
with per-branch IQN quantile-τ schedules derived from Q-distribution skew.
q_skew_kurtosis_update: single-block 4-thread, reads save_q_online[B×13],
two-pass central moments → skew clamped [-3,+3] + ex_kurt clamped [-3,+30];
writes scratch[171..179). EPS_DIV=1e-12 (Invariant 1 anchor). No atomicAdd.
pearl_5_iqn_tau_update: single-block 4-thread, shifts symmetric default τ
{0.05,0.25,0.5,0.75,0.95} by skew×SKEW_SHIFT=0.05, clamps to [0.01,0.99];
writes scratch[179..199). SKEW_SHIFT and envelope are Invariant 1 anchors.
launch_sp5_pearl_5_iqn_tau fires both kernels + 20 apply_pearls_ad calls
(ALPHA_META=1e-3) → ISV[IQN_TAU_BASE=250..270). SP5_SCRATCH_TOTAL 171→199.
StateResetRegistry +1 FoldReset entry (sp5_iqn_tau, ISV[250..270)).
training_loop.rs wired after Pearl 4 with tracing::warn on error.
Two GPU-only unit tests (10+11): zero-skew symmetric default + left-skew
floor clamp. Module docstring updated A1-A5.
No consumer migration — Layer A additive only per spec.
cargo check -p ml --offline clean (11 pre-existing warnings, none new).
cargo test --no-run clean.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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