feat(exporter): SEO/Perf small improvements (canonical, lazy/async hero, dark token) + tests

lib/exporter/html.ts

@@ -108,7 +108,7 @@ function renderHero(section: { props: { heading: string; subheading?: string; im
           ${subheading ? `<p>${escapeHtml(subheading)}</p>` : ''}
         </div>
         <div class="media">
-          <img src="${escapeHtml(imageUrl)}" alt="${escapeHtml(heading)}" />
+          <img src="${escapeHtml(imageUrl)}" alt="${escapeHtml(heading)}" loading="lazy" decoding="async" />
         </div>
       </div>
     </section>
@@ -237,6 +237,7 @@ function buildHead(page: Page) {
   return `
     <meta charset="utf-8" />
     <meta name="viewport" content="width=device-width, initial-scale=1" />
+    <link rel="canonical" href="/" />
     <title>${escapeHtml(page.seo.title)}</title>
     <meta name="description" content="${escapeHtml(page.seo.description)}" />
     <meta property="og:title" content="${escapeHtml(page.seo.title)}" />
@@ -272,6 +273,7 @@ function buildCss(page: Page) {
   .testimonial blockquote{ margin:0; font-style:italic; color:#1f2937 }
   .testimonial figcaption{ color:#475569; margin-top:.25rem }
   @media (max-width: 640px){ .container{ padding: 1rem } h1{ font-size: 1.5rem } }
+  @media (prefers-color-scheme: dark){ body{ color:#e5e7eb; background:#0b1020 } .feature-item{ background:#0f172a; border-color:#1f2937 } }
   @media (prefers-reduced-motion: reduce){ *{ transition: none !important; animation: none !important } }
   `
 }