Function localizedSectionTitle

Resolve the localised journalist title for a canonical section slug.

Resolution order:

1. Explicit journalist / deep-dive override map.
2. per-document-intelligence → reader-guide perDocLabel (reused).
3. Backing-artifact title via ARTIFACT_TITLE_I18N.

Returns undefined when the slug has no curated localisation, letting the caller fall back gracefully to the (English) heading text.