4-tab tablist + WAI-ARIA grid for the active category only. Switching
tabs unmounts the previous category's cells so the DOM never holds more
than ~140 buttons at once (animals is the worst case). Cells use roving
tabindex (first cell tabindex=0, rest -1) per the grid pattern; arrow
keys / Home / End walk focus cell-by-cell, the tablist receives a single
Tab stop.
Covers EP-U-01..05 (tab count + role, default selected, click→onSelect,
tab switch re-renders grid, ArrowRight roving focus).
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
