Refactor to PSR-4: rename files and switch namespace

- Rename files to PascalCase: Product_Type → ProductType, Cart_Handler →
  CartHandler, Product_Selector → ProductSelector, Stock_Manager →
  StockManager, Admin/Product_Data → Admin/ProductData
- Switch namespace from WC_Composable_Product to Magdev\WcComposableProduct
- Update all cross-references in PHP, CSS, JS, translations, and docs

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

composer.json

@@ -15,7 +15,7 @@
     },
     "autoload": {
         "psr-4": {
-            "WC_Composable_Product\\": "includes/"
+            "Magdev\\WcComposableProduct\\": "includes/"
         }
     },
     "config": {