Connectivity test: add a Mexico country list

We ship country-focused connectivity lists (Iran, Russia, China, Brazil, India…) so people can quickly check which services matter for that country. Mexico is Latin America's second-largest internet market and only Brazil is covered so far — if you're familiar with the Mexican internet, you know better than we do which sites matter.

Same recipe as #405: id: 'mexico', emoji , ≥10 members mixing global services relevant locally with major Mexican platforms (e.g. Mercado Libre México, Liverpool, Coppel, El Universal, gob.mx, Rappi…), names in every full locale pack (en, zh, fr, pt-BR, ru), favicons committed via pnpm fetch-favicons.

pnpm check green; comment to claim; PR targets dev.