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-data.php

@@ -0,0 +1,35 @@
+<?php
+/**
+ * Minimal WC_Data stub for unit testing.
+ */
+class WC_Data {
+    protected $id = 0;
+    protected $data = [];
+    protected $meta_data = [];
+
+    public function __construct($id = 0) {
+        if (is_numeric($id) && $id > 0) {
+            $this->id = (int) $id;
+        }
+    }
+
+    public function get_id() {
+        return $this->id;
+    }
+
+    public function set_id($id) {
+        $this->id = (int) $id;
+    }
+
+    public function get_meta($key, $single = true, $context = 'view') {
+        return $this->meta_data[$key] ?? ($single ? '' : []);
+    }
+
+    public function update_meta_data($key, $value, $meta_id = 0) {
+        $this->meta_data[$key] = $value;
+    }
+
+    public function save() {
+        return $this->get_id();
+    }
+}