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Emitting reciprocal hreflang only when both languages exist — Aulvem's i18n customisation
Astro's i18n routing doesn't emit hreflang on its own, and a naive 'always emit all locales' implementation points at 404s for one-sided posts. Aulvem uses Content Collections helpers to emit alternates only for languages that actually have a published entry, then handles the reciprocal and x-default rules on top.
