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>

includes/Admin/Settings.php

@@ -2,10 +2,10 @@
 /**
  * Admin Settings
  *
- * @package WC_Composable_Product
+ * @package Magdev\WcComposableProduct
  */
 
-namespace WC_Composable_Product\Admin;
+namespace Magdev\WcComposableProduct\Admin;
 
 defined('ABSPATH') || exit;