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-cart.php

@@ -0,0 +1,15 @@
+<?php
+/**
+ * Minimal WC_Cart stub for unit testing.
+ */
+class WC_Cart {
+    protected $cart_contents = [];
+
+    public function get_cart() {
+        return $this->cart_contents;
+    }
+
+    public function set_cart_contents($contents) {
+        $this->cart_contents = $contents;
+    }
+}