fix: flush static buffer in conditionals and correct helpers import path

- Flush static buffer at end of each conditional branch (if/else/else-if)
  to ensure content is rendered inside the correct blocks
- Add helpers_path parameter to zig_codegen.generate() for correct
  relative imports in nested directories (e.g., '../helpers.zig')
- Fix build.zig to use correct CLI path (src/tpl_compiler/main.zig)
- Export zig_codegen from root.zig for CLI module usage

Bump version to 0.3.11

examples/demo/views/pages/home.pug

@@ -1,5 +1,5 @@
 extends ../layouts/base.pug
-
+include ../mixins/alerts.pug
 block title
   title #{title} | Pugz Store
 
@@ -54,3 +54,6 @@ block content
           .category-icon H
           h3 Home Office
           span 12 products
+
+  if alert_message
+    +alert_error(alert_message)