fix(serve): route /<slug>/ and /<slug> to index.html

Local serve was returning 404 on /socket-packageurl-js/ — the slug-
prefixed root that GH Pages serves the site at and the home link
(<a href='/'>) falls back to when base-path rewrite runs. Only the
bare /, /<slug>/part/<n>, and /<slug>/documents shapes were handled.

Add an explicit match for /<slug> and /<slug>/ → index.html so the
dev server matches production URL shape.

Author: jdalton

scripts/walkthrough.mts

@@ -810,12 +810,18 @@ function readSlug(): string {
 
 function routeToFile(slug: string, urlPath: string): string | undefined {
   // /                           → index.html
+  // /<slug>  or  /<slug>/       → index.html (slug-prefixed root, same as the
+  //                               URL GH Pages serves the site at; matches
+  //                               the flat file name emitted by meander)
   // /<slug>/part/<n>            → walkthrough-part-<n>.html
   // /<slug>/documents           → documents.html
   // anything else               → as-is (e.g. /walkthrough.css)
   if (urlPath === '/' || urlPath === '') {
     return 'index.html'
   }
+  if (urlPath === `/${slug}` || urlPath === `/${slug}/`) {
+    return 'index.html'
+  }
   const partMatch = new RegExp(`^/${slug}/part/(\\d+)/?$`).exec(urlPath)
   if (partMatch) {
     return `walkthrough-part-${partMatch[1]}.html`