feat(gallery): add R20 drum roller selector

An equalizer-style tone selector pair as inline SVG (reference filed in the catalogue's working directory): each numbered drum shows through a tall window with curvature shading, and the center value sits on an inverted chip. The drums drag independently with a short roll transition.

This differs from the thumbwheel, which shows one number in a separate window. Here the visible number strip is itself the control face, like the source hardware.

I verified both drums in headless Chrome: independent drags roll the strips and update the chips, no console exceptions.

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