Add ViewEngine for easy server integration

- ViewEngine manages views directory with path resolution
- Auto-loads mixins from views/mixins/ directory
- Simplifies template paths (relative to views dir, auto-adds extension)
- Updated example app to use ViewEngine
- Added example mixins (buttons.pug, cards.pug)
- Updated CLAUDE.md with ViewEngine documentation

src/examples/app_01/views/mixins/cards.pug

@@ -0,0 +1,11 @@
+mixin card(title)
+  .card
+    .card-header
+      h3= title
+    .card-body
+      block
+
+mixin card-simple(title, body)
+  .card
+    h3= title
+    p= body