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

src/root.zig

@@ -8,6 +8,7 @@
 pub const pug = @import("pug.zig");
 pub const view_engine = @import("view_engine.zig");
 pub const template = @import("template.zig");
+pub const parser = @import("parser.zig");
 
 // Re-export main types
 pub const ViewEngine = view_engine.ViewEngine;