Update on "Improve complex lerp performance"

The complex lerp kernel uses `std::abs(z) < 0.5` which involves
computing a sqrt. Instead compare the square against 0.25 has much
lower latency and so performs much better overall.

In a simple timeit benchmark I see more than 10x speedup on CPU for a 4096
element complex lerp, from 84 us to 6.7 us.

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