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>

assets/css/admin.css

@@ -1,7 +1,7 @@
 /**
  * Admin Styles for Composable Products
  *
- * @package WC_Composable_Product
+ * @package Magdev\WcComposableProduct
  */
 
 /* Hide composable panel by default */

assets/css/frontend.css

@@ -1,7 +1,7 @@
 /**
  * Frontend Styles for Composable Products
  *
- * @package WC_Composable_Product
+ * @package Magdev\WcComposableProduct
  */
 
 .wc-composable-product-selector {

assets/js/admin.js

@@ -1,7 +1,7 @@
 /**
  * Admin JavaScript for Composable Products
  *
- * @package WC_Composable_Product
+ * @package Magdev\WcComposableProduct
  */
 
 (function($) {

assets/js/frontend.js

@@ -1,7 +1,7 @@
 /**
  * Frontend JavaScript for Composable Products
  *
- * @package WC_Composable_Product
+ * @package Magdev\WcComposableProduct
  */
 
 (function($) {