schema.html always read the author from .Site.Params.Author, so the
structured-data author was the site-level author (or empty) regardless of a
post's `authors` front matter — even though the theme supports multiple
authors via data/authors/.
Resolve the first `authors` key against data/authors/ and emit that author's
name, image and sameAs (from their social/links), falling back to the
site-level author. Single-author output is unchanged apart from the additive
image and sameAs fields.
Fixes#2943
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
