feat: add template inheritance (extends/block) support

- ViewEngine now supports extends and named blocks
- Each route gets exclusive cached AST (no shared parent layouts)
- Fix iteration over struct arrays in each loops
- Add demo app with full e-commerce layout using extends
- Serve static files from public folder
- Bump version to 0.3.0

examples/demo/views/partials/header.pug

@@ -0,0 +1,11 @@
+header.header
+  .container
+    .header-content
+      a.logo(href="/") Pugz Store
+      nav.nav
+        a.nav-link(href="/") Home
+        a.nav-link(href="/products") Products
+        a.nav-link(href="/about") About
+      .header-actions
+        a.cart-link(href="/cart")
+          | Cart (#{cartCount})