@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/section_link.h

@@ -0,0 +1,49 @@
+// @link :section -- a link to a section by the TEXT of its heading.
+//
+// Decided by Andy, 2026-10-12.  :section (argtype section_link) written
+// alone says the heading is in the CURRENT file; written with a value, the
+// value is the basename (no directory, no extension) of the file of the
+// same @document that has it.  So a link names exactly one file, and a
+// title shared between files is never ambiguous.  Titles match with
+// klammer markup removed and whitespace collapsed to one space,
+// case-sensitive.
+//
+// Resolution happens here, when @document assembles its files -- the only
+// place every heading is known -- from two markers the klammers emit:
+//   Link      __seclink__<title>__KTSL__<file>__   (in the href / \hyperlink;
+//             <file> empty for the current one)
+//   Section   %__sectionlink__<id>__KTSL__<basename>__KTSL__<title>__   (tex only;
+//             the html pass reads the heading elements themselves, which
+//             carry their ids after insert_ids())
+// KTSL separates the fields because a title or a basename may contain a
+// single underscore (md_to_sks_test).  A file the document does not have, a
+// title the file does not have, and a title it has more than once are
+// errors, never fallbacks (the commands do not warn and continue).
+#ifndef SECTION_LINK_H
+#define SECTION_LINK_H
+
+#include <string>
+#include <vector>
+
+struct Section_target {
+    std::string title {};       // normalized
+    std::string basename {};    // the file's basename, "" for :text content
+    std::string id {};          // the anchor: html id, or tex \hypertarget name
+};
+
+// The comparison form of a heading's text: tags and TeX control sequences
+// removed, braces dropped, whitespace collapsed, trimmed.
+std::string normalize_section_title(const std::string& s);
+
+// The target a :section link names, or an Argument_error that says why
+// there is none.  FILE is the :section value ("" for the current file),
+// WHERE the basename of the file holding the link ("" for the document's
+// :text) and FILES the document's basenames.  At assembly the link's own
+// position is gone, so the file is the location an error can give
+// (Locator::none(), never the C++ site).
+Section_target resolve_section_target(
+    const std::vector<Section_target>& table, const std::string& title,
+    const std::string& file, const std::string& where,
+    const std::vector<std::string>& files);
+
+#endif