fn quat_pow(g_bhj3: Tensor<4>, k: f32) -> Tensor<4>Expand description
exp(k·g/2) — the per-step rotation quaternion raised to the real power
k, built like quat_from_scaled_axis but with the half-angle reduced
mod 2π so large k stays fp-accurate (the reduction is value-exact and
the wrap offset is detached, so d/dg matches the unrolled product).