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/layouts/base.pug

@@ -0,0 +1,28 @@
+doctype html
+html(lang="en")
+  head
+    meta(charset="UTF-8")
+    meta(name="viewport" content="width=device-width, initial-scale=1.0")
+    link(rel="stylesheet" href="/css/style.css")
+    block title
+      title Pugz Store
+  body
+    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})
+
+    main
+      block content
+
+    footer.footer
+      .container
+        .footer-content
+          p Built with Pugz - A Pug template engine for Zig