:root{
  --u:4px;          /* base unit */
  --key:40px;       /* key face */
  --gutter:8px;     /* seam — one value, everywhere, forever */
  --pitch:48px;     /* key 40 + gutter 8 */
  --pad:16px;       /* part padding */
  --clearance:6px;  /* control to well wall */
  --dock-snap:12px; /* magnetic dock distance */

  /* radius never scales */
  --r-inlay:2px;
  --r-key:3px;
  --r-panel:5px;
}