chore(seed): neutralize stale 'Cursada Demo' literal at seed-demo.ts:152 -> 'Demo Periodo' (removes latent re-break if the real 2026 periodo is ever deactivated) — source-hardening for VENUS-124

Low-pri. Latent-only: fires only if the active real periodo is soft-deleted without replacement, then seed would mint a new demo-labeled periodo. 1-line literal change. Class-S, security-inert, reversible.