grad_norm_buf is single-element f32; p99 == the element. Kernel copies
to scratch slot 38 with __threadfence_system; host applies Pearls A+D
to update ISV[GRAD_CLIP_BOUND_INDEX=168]. Trivial pattern — establishes
the launcher template for Mech 6's eventual ISV read in Layer B.
GPU test verifies first-observation Pearl A replacement (input=42.0 →
scratch[38]=42.0 bit-identical → new_x_mean=42.0, x_lag=42.0, both
variances stay zero) and that all non-target scratch slots remain 0.
No consumer wired yet. Behaviour unchanged. cargo check --lib --tests clean.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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