index_add with duplicate indices double-applies deltas — e.g. if index i
is sampled twice with delta -0.5, priority goes 1.0 + (-0.5) + (-0.5) = 0.0,
causing NaN in importance sampling weights. Post-clamp to [epsilon, 1e6]
ensures priorities stay positive.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>