A 440 Hz sine at sr=8192 with N=8192 makes bin index equal Hz exactly, so
the fft() peak bin must be 440 with zero tolerance.
ifft(fft([1,2,3,4])) must return 1,2,3,4 exactly.
The stft(1024/256) magnitude spectrogram below must show one
horizontal line at bin 55 (440 Hz at 8 Hz/bin), and istft()
must reconstruct the interior of the signal to maxErr < 1e-4.
Same asserts run headless in examples/node/xa-fft.mjs.