feat: add interactive homepage layout lab

exampleSite/content/layout-lab/_index.md

@@ -0,0 +1,13 @@
+---
+title: "Homepage layout lab"
+description: "Try Blowfish's built-in homepage layouts without changing your site configuration."
+type: "layout-lab"
+---
+
+{{< lead >}}
+Choose a layout below to preview how the same content behaves across Blowfish's built-in homepage patterns.
+{{< /lead >}}
+
+{{< alert "circle-info" >}}
+This is an example-site playground. Your own site selects a homepage layout with `homepage.layout` in `params.toml`.
+{{< /alert >}}