Feed a pure 440 Hz tone into the analyzer and the spectrogram below should
show exactly one bright horizontal line — at 440 Hz and nowhere else.
Because the tone's frequency is known in advance, every number the FFT and
STFT report can be checked against the truth. Technically: at sr=8192 with
N=8192, bin index equals 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 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.