Link legacy routes (/alumnos, /docentes, /administrar/usuarios) into the new unified /usuarios directory + eventual consolidation decision

Follow-up from MARS /usuarios unified-directory ship (@ea68430/v2.56.0, 2026-07-23). Old routes deliberately left untouched per Elazar ruling 6 (no redirects at ship time). Next: cross-link legacy pages to the new component, then decide whether/when to consolidate/retire the old routes.