@link :section links to a section by its heading, with an alone value (from dev 831455f36fcd)

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>

sks/document/document_class.h

@@ -72,6 +72,9 @@ public:
     std::map<std::string, std::string> id_to_caption_number();
     std::map<std::string, std::vector<std::pair<std::string, std::string>>> html_references();
     void resolve_html_references();
+    void resolve_section_links();                               // html, @link :section
+    std::string resolve_tex_section_links(
+        const std::vector<std::pair<std::string, std::string>>& pieces);  // (basename, tex)
     std::string single_page_toc();
     elements_t page(const std::string& body, const std::string& output_dir, int max_level);