Add a function to extract ToC entries from HTML

src/lib/components/TableOfContents.svelte

@@ -1,4 +1,5 @@
 <script lang='ts'>
+  import { page } from '$app/state';
   import type { PublicationTree } from '$lib/data_structures/publication_tree';
   import type { TocEntry } from '$lib/data_structures/table_of_contents';
   import { getContext } from 'svelte';
@@ -10,6 +11,35 @@
   // If single event, build the table of contents from the rendered HTML.
   // Each rendered `<h>` should receive an entry in the ToC.
 
+  function buildTocFromDocument(parentElement: HTMLElement): void {
+    const entries: TocEntry[] = [];
+    const currentPathname = page.url.pathname;
+
+    parentElement
+      .querySelectorAll<HTMLHeadingElement>(
+        'h1, h2, h3, h4, h5, h6'
+      )
+      .forEach((header) => {
+        const title = header.textContent?.trim();
+        const id = header.id;
+
+        // Only create an entry if the header has an ID and a title.
+        if (id && title) {
+          const href = `${currentPathname}#${id}`;
+
+          const tocEntry: TocEntry = {
+            title,
+            href,
+            expanded: false, // Default to not expanded; can be changed as needed.
+            children: null,  // These are leaf entries from a flat scan of headers.
+          };
+          entries.push(tocEntry);
+        }
+      });
+
+    // TODO: Update ToC state within the component.
+  }
+
   let toc = $state<TocEntry[]>([]);
 
 	const publicationTree = getContext('publicationTree') as PublicationTree;