PLUTO-129 Hardening: roster-add forms allow adding a fraud-table membership into a NON-current/closed period — isPeriodoValid checks only deletedAt IS NULL, not isCurrent. Affects BOTH the new docente-detail add-form (PLUTO-128) AND the already-shipped list-page dialog (docentes/page.tsx) — PARITY, not a 128 regression. If cross-period roster-add is NOT an intended workflow (pre-stage future / correct past period), gate both forms on isCurrent. PENDING Elazar's intent call (is cross-period add a feature?). Surfaced by audit on PLUTO-128 pre-push review.