Fix infinite loop in lexer when parsing attribute expressions with operators

The lexer would hang when encountering operators like + in attribute values
(e.g., class="btn btn-" + type). Added scanAttrValuePart to handle
expression continuation with operators (+, -, *, /).

Also cleaned up debug prints from view_engine.zig and demo/main.zig.

src/examples/demo/views/layout.pug

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