A section link names the TEXT of a heading: :section alone means the current file, a value names the file of the same @document that has it. Resolved by @document at assembly in html and tex; errors for a missing file, a missing title, a doubled title and a link outside a @document. The Markdown converter reads a Markdown anchor as a section link to the heading its slug names. Also from dev: the converter's four silent defects fixed and --chapter (9ae2c4cd), and the engine's alone-value error text. Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>