feat(sp4): Task A6 — atom_pos_p99 producer × 4 branches
Single kernel parameterized by branch slice. Launcher loops 0..4,
launching once per branch with distinct scratch slot (1..5). Phase 2
applies Pearls A+D host-side per branch, writing to ISV[ATOM_POS_BOUND[
branch]] + wiener_state. Same end-to-end pattern as Task A5.
GPU test verifies per-branch independence: 4 scales of |N(0,1)| samples
(1×, 10×, 100×, 1000×) → 4 distinct p99 values within 5% tolerance.
No consumer wired yet. Behavior unchanged. cargo check --lib --tests clean.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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