fix: add scoped error logging for lexer/parser errors

- Add std.log.scoped(.pugz) to template.zig and view_engine.zig
- Log detailed error info (code, line, column, message) when parsing fails
- Log template path context in ViewEngine on parse errors
- Remove debug print from lexer, use proper scoped logging instead
- Move benchmarks, docs, examples, playground, tests out of src/ to project root
- Update build.zig and documentation paths accordingly
- Bump version to 0.3.1

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})