Add PHPUnit test suite, PSR-4 refactor, lint+test CI jobs (v1.3.1)

- 57 unit tests covering ProductType, StockManager, CartHandler, Plugin,
  Admin/ProductData, Admin/Settings using Brain Monkey + Mockery
- WooCommerce class stubs for testing without WP installation
- PHP lint and test jobs in release workflow (test gate blocks release)
- PSR-4 namespace change: WC_Composable_Product -> Magdev\WcComposableProduct
- PascalCase filenames for all classes under includes/

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

tests/stubs/class-wc-order-item-product.php

@@ -0,0 +1,28 @@
+<?php
+/**
+ * Minimal WC_Order_Item_Product stub for unit testing.
+ */
+class WC_Order_Item_Product extends WC_Data {
+    protected $product = null;
+    protected $quantity = 1;
+
+    public function get_product() {
+        return $this->product;
+    }
+
+    public function set_product($product) {
+        $this->product = $product;
+    }
+
+    public function get_quantity() {
+        return $this->quantity;
+    }
+
+    public function set_quantity($quantity) {
+        $this->quantity = $quantity;
+    }
+
+    public function add_meta_data($key, $value, $unique = false) {
+        $this->meta_data[$key] = $value;
+    }
+}