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-settings-page.php

@@ -0,0 +1,29 @@
+<?php
+/**
+ * Minimal WC_Settings_Page stub for unit testing.
+ */
+class WC_Settings_Page {
+    protected $id = '';
+    protected $label = '';
+
+    public function __construct() {
+    }
+
+    public function get_id() {
+        return $this->id;
+    }
+
+    public function get_label() {
+        return $this->label;
+    }
+
+    public function get_settings() {
+        return [];
+    }
+
+    public function output() {
+    }
+
+    public function save() {
+    }
+}