Function deriveBriefSeoOverrides

Derive <title> / <meta description> / keyword-entity overrides from the executive-brief markdown adjacent to article.md. Pure function — no I/O, no clock.

Resolution order:

1. For non-EN, prefer RenderArticleInput.localizedBriefMarkdown via extractLocalizedBriefSeo. Title and description are resolved independently: a banned title with a clean BLUF still localizes the description, and a clean title with an empty BLUF still localizes the title.
2. Whatever field is still null after step 1 falls through to the English brief — title via readFirstHeading → cleanArticleTitle; description via composeRichDescription ∥ readBlufParagraph ∥ readFirstParagraph, capped by per-language SERP window in truncateToSentenceBoundary.
3. Entities are mined from the brief (localized first, EN fallback) — universal-Swedish identifiers (HD03267, JuU/SfU) carry across locales.

Returns { title: undefined, description: undefined, entities: [] } when no brief markdown is provided so the head-metadata helper falls back to the legacy frontmatter-only audit path (covers the 278 pre- 2026-03-26 news/*-en.html files whose source analysis/daily/ directories have been deleted).

Exported for testability.