@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/link/js/link.js

@@ -1,8 +1,19 @@
 
+// In-text links to a section on another (or the same) page of a book:
+// <a class="pagelink" data-page="BASENAME" data-id="ID">, which @document
+// emits for @link ... :section @ (document_html.cpp, resolve_section_links).
+// go_to_section() swaps in the embedded page and scrolls to the id, and
+// calls this again for the links of the page it just showed.  Each anchor
+// is bound once: a page shown twice is the same nodes bound twice otherwise.
 function set_link_listeners() {
     Array.from(document.getElementsByClassName("pagelink")).forEach(
         function (elt) {
+            if (elt.getAttribute("data-bound")) {
+                return;
+            }
+            elt.setAttribute("data-bound", "true");
             elt.addEventListener("click", function(event) {
+                event.preventDefault();
                 let basename = elt.getAttribute("data-page");
                 let id = elt.getAttribute("data-id");
                 go_to_section(elt, basename, id);
@@ -10,3 +21,4 @@ function set_link_listeners() {
         });
 }
 
+window.addEventListener("load", set_link_listeners);