feat(gallery): add R22 three-position slideHEADmain

A detented slide switch after a drum machine's variation control (reference filed in the catalogue's working directory): a chrome pill parks at A, AB, or B, the active legend lights, and the LED pair below reports honestly — A or B lights its own dot, AB lights both. Clicking a position moves the pill with a short settle.

The kit had toggles, segmented buttons, and continuous sliders, but no detented multi-position slide.

I verified all three positions in headless Chrome: the pill parks, legends relight, and the LED count matches the position (2 for AB, 1 otherwise), no console exceptions.

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