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-product.php

@@ -0,0 +1,88 @@
+<?php
+/**
+ * Minimal WC_Product stub for unit testing.
+ */
+class WC_Product extends WC_Data {
+    protected $supports = [];
+    protected $data = [
+        'name' => '',
+        'price' => '',
+        'regular_price' => '',
+        'status' => 'publish',
+    ];
+
+    public function get_type() {
+        return 'simple';
+    }
+
+    public function get_name($context = 'view') {
+        return $this->data['name'] ?? '';
+    }
+
+    public function get_price($context = 'view') {
+        return $this->data['price'] ?? '';
+    }
+
+    public function get_regular_price($context = 'view') {
+        return $this->data['regular_price'] ?? '';
+    }
+
+    public function get_price_html() {
+        return '';
+    }
+
+    public function get_permalink() {
+        return '';
+    }
+
+    public function get_image_id($context = 'view') {
+        return 0;
+    }
+
+    public function get_stock_quantity($context = 'view') {
+        return null;
+    }
+
+    public function get_stock_status($context = 'view') {
+        return 'instock';
+    }
+
+    public function get_children() {
+        return [];
+    }
+
+    public function is_type($type) {
+        return $this->get_type() === $type;
+    }
+
+    public function is_purchasable() {
+        return true;
+    }
+
+    public function is_in_stock() {
+        return true;
+    }
+
+    public function managing_stock() {
+        return false;
+    }
+
+    public function backorders_allowed() {
+        return false;
+    }
+
+    public function is_sold_individually() {
+        return false;
+    }
+
+    public function set_price($price) {
+        $this->data['price'] = $price;
+    }
+
+    public function set_stock_quantity($quantity) {
+    }
+
+    public function supports($feature) {
+        return in_array($feature, $this->supports, true);
+    }
+}