feat(gallery): add R31 radar sweepHEADmain

The rotating scan after a radar PPI scope (reference filed in the catalogue's working directory): a beam circles inside a numbered bearing ring, an afterglow wedge trails it, contacts bloom as the beam passes and fade over the next half turn, and range rings sit under it all. Clicking marks the current bearing. The kit's round CRT draws traces; nothing rotated a scan until this.

I verified it in headless Chrome: the sweep rotates, the mark click captures the live bearing, no console exceptions.

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