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/mixins/rating.pug

@@ -0,0 +1,13 @@
+//- Star rating display
+//- Parameters:
+//-   stars: number of stars (1-5)
+
+mixin rating(stars)
+  .stars
+    - var i = 1
+    while i <= 5
+      if i <= stars
+        span.star.star-filled
+      else
+        span.star.star-empty
+      - i = i + 1