WGSL leaves the result of an overflowing pow indeterminate, so clamping
after it did not guarantee the CPU's 1e30 bound on every backend. A curve
past the bound (log2|t| / gamma > 99.7) now saturates to +-1e30 without
calling pow; between 1e30 and 2^99.7 the clamp gives the same 1e30, so the
GPU matches the CPU everywhere.