fix: render heading IDs so anchor links resolve

marked v12 dropped the headerIds option, so headings rendered with no
id attributes and links like [Releases](#releases-and-contributing)
silently failed to scroll. Add a heading renderer that emits a
GitHub-style slug id, with a numeric suffix for duplicates within a
document.

Fixes #390.

public/script/utils.js

@@ -143,6 +143,19 @@ function parseGithubUrl(url) {
   }
 }
 
+// GitHub-style heading slug: lowercase, drop punctuation other than `-_`,
+// collapse runs of whitespace into a single dash. Used so anchor links like
+// `[Releases](#releases-and-contributing)` actually jump to the heading
+// (marked v12 dropped `headerIds`, so headings now have no id by default).
+function slugifyHeading(text) {
+  return String(text)
+    .toLowerCase()
+    .replace(/<[^>]+>/g, "")
+    .replace(/[^\p{L}\p{N}\s_-]/gu, "")
+    .trim()
+    .replace(/\s+/g, "-");
+}
+
 function renderMD(md, baseUrlValue) {
   marked.use(
     markedEmoji({
@@ -162,6 +175,15 @@ function renderMD(md, baseUrlValue) {
     return rendererLink.call(this, href, title, text);
   };
 
+  const slugCounts = {};
+  renderer.heading = function (text, level, raw) {
+    const base = slugifyHeading(raw || text);
+    const n = slugCounts[base] || 0;
+    slugCounts[base] = n + 1;
+    const id = n === 0 ? base : `${base}-${n}`;
+    return `<h${level} id="${id}">${text}</h${level}>\n`;
+  };
+
   marked.setOptions({
     renderer: renderer,
     pedantic: false,