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

playground/examples/extend-layout.pug

@@ -0,0 +1,10 @@
+html
+  head
+    h1 My Site - #{title}
+    block scripts
+      script(src='/jquery.js')
+  body
+    block content
+    block foot
+      #footer
+        p some footer content