@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_html.cpp

@@ -1,3 +1,4 @@
+#include "section_link.h"
 #include <iomanip>
 #include "util.h"
 #include "error.h"
@@ -315,6 +316,7 @@ void Document_class::process_input_files()
         //if (true || !in_modification_order(inpath, cached_filename, m_no_cache)) {
         if (m_no_cache || cache_requires_update(m_cache_dir, inpath.string(), basename)) {
             Machine M = m_machine;
+            M.m_state.set("K_document_file", basename);   // see document_latex.cpp
             M.read(inpath);
             html_text = trim(M.apply("html"));
             html_text = trim(html::make_paragraphs(html_text)) + "\n";
@@ -686,6 +688,54 @@ elements_t Document_class::page(const std::string& body_text, const std::string&
     return page;
 }
 
+// @link :section (section_link.h).  Every heading element that carries an
+// id -- numbered or not, so @h1..@h7 and @part are linkable too, which is
+// why this reads the elements rather than the headings table that
+// insert_section_numbers() builds from the numbered ones -- is a target;
+// the Link klammer's href marker is resolved against them.  A single page
+// gets href="#id".  A book's pages are embedded in index.html and shown by
+// go_to_section(), so a link there is the "pagelink" anchor that
+// sks/link/js/link.js binds: data-page and data-id, with the app's own
+// "#page#id" hash as the href so the location survives without the script.
+// Before 2026-10-12 html had no resolver and the marker was the href.
+void Document_class::resolve_section_links()
+{
+    static const std::regex heading_rgx(R"(<(h[1-7]|kt-chapter|kt-part)\b([^>]*)>(.*?)</\1>)");
+    static const std::regex link_rgx(R"(href="__seclink__(.*?)__KTSL__(.*?)__")");
+    std::vector<Section_target> table {};
+    strings_t files {};
+    for (const auto& [basename, text] : m_file_components) {
+        files.push_back(basename);
+        for (std::sregex_iterator it(text.cbegin(), text.cend(), heading_rgx), end; it != end; ++it) {
+            const std::smatch& m = *it;
+            std::string attrs = m[2];
+            std::smatch id_match {};
+            if (std::regex_search(attrs, id_match, id_attr_rgx)) {
+                table.push_back({normalize_section_title(m[3]), basename, id_match[1]});
+            }
+        }
+    }
+    const bool book = m_structure == docstruct_t::book;
+    for (auto& [basename, text] : m_file_components) {
+        std::string result {};
+        auto begin = text.cbegin();
+        for (std::sregex_iterator it(text.cbegin(), text.cend(), link_rgx), end; it != end; ++it) {
+            const std::smatch& m = *it;
+            result.append(begin, m[0].first);
+            const Section_target t = resolve_section_target(table, m[1], m[2], basename, files);
+            if (book) {
+                result += "href=\"#" + t.basename + "#" + t.id + "\" class=\"pagelink\" data-page=\""
+                    + t.basename + "\" data-id=\"" + t.id + "\"";
+            } else {
+                result += "href=\"#" + t.id + "\"";
+            }
+            begin = m[0].second;
+        }
+        result.append(begin, text.cend());
+        text = result;
+    }
+}
+
 std::string Document_class::make_single_html_page(const std::string& output_directory)
 {
     (void)K::log(3);
@@ -787,6 +837,7 @@ std::string Document_class::html()
     }
     auto headings = insert_section_numbers();
     insert_section_numbers("___NUM2___", false);
+    resolve_section_links();
     insert_html_caption_numbers();
     resolve_html_references();
     if (m_structure != docstruct_t::book) {