Single-select subcascade radio can't be deselected (SOP-14)

SOP-14 (Agustin Pereira): single-select subcascade radio groups (non-MULTI_GROUPS keys, e.g. 'caninos' surface on retained-canine cirugía) are push-only — native radio onChange doesn't fire on re-click of the selected option + no clear affordance, so a mis-click can't be undone. Fix (class-wide, client-only, write-shape unchanged): re-click-to-clear via clearSubcascadeKey + a visible 'Limpiar selección' affordance on every single-select subcascade group. Class-S/solo.