Teach DFT orthogonality with overlaid waves and product stems

A student who kept asking why a DFT 'sends out its own wave' only got it after seeing a two-column chart: top row overlays the test wave and the tone, bottom row shows their pointwise product as stems, with the sum as the bar height. ASCII drawings were not enough for overlaid curves, and the student explicitly asked for real charts. Small gotcha when labelling that sum in matplotlib: f'{x:.0f}' on a tiny negative float prints '-0', so use int(round(x)).