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>

wc-composable-product.php

@@ -60,7 +60,7 @@ function wc_composable_product_init() {
     load_plugin_textdomain('wc-composable-product', false, dirname(WC_COMPOSABLE_PRODUCT_BASENAME) . '/languages');
 
     // Initialize main plugin class
-    WC_Composable_Product\Plugin::instance();
+    Magdev\WcComposableProduct\Plugin::instance();
 }
 // Use woocommerce_init to ensure all WooCommerce classes including settings are loaded
 add_action('woocommerce_init', 'wc_composable_product_init');