Function composeContextualExtendedDescription

Build a per-article extendedDescription (used for og:description, Twitter cards, and AI-overview surfaces) that is always ≥ DESCRIPTION_MAX_LENGTH characters whenever the editorial source paragraph is too short to satisfy truncateExtendedDescription on its own.

This is the only code path that surfaces the localized labels.reader framing — the short <meta description> no longer carries it (see comment in composeContextualDescription). The structure is: <base> <Date: YYYY-MM-DD.> <Context: …> <reader>, passed through truncateExtendedDescription (300-char max with a 200-char min) so it occupies the Open Graph / Discover budget without exceeding it.