Release version 1.1.16 - Singleton pattern for settings page

Fixed persistent duplicate settings page rendering by implementing proper
singleton pattern for admin class and caching settings instance.

Fixes:
- Settings page still appearing twice despite v1.1.15 fix
- Multiple instantiation of WC_TPP_Admin class
- Duplicate creation of WC_TPP_Settings instances

Changes:
- Implemented singleton pattern for WC_TPP_Admin class
- Added private static $instance property with get_instance() method
- Made WC_TPP_Admin constructor private
- Added static $settings_instance property to cache settings page
- Modified add_settings_page() to check and reuse cached settings instance
- Changed instantiation from new WC_TPP_Admin() to WC_TPP_Admin::get_instance()

Technical Details:
- Ensures only one WC_TPP_Admin instance exists throughout plugin lifecycle
- Prevents duplicate filter registrations even if woocommerce_get_settings_pages called multiple times
- Settings page object created once and reused on subsequent filter calls
- Follows WordPress/WooCommerce best practices for singleton implementation

Updated Files:
- includes/class-wc-tpp-admin.php (singleton pattern implementation)
- wc-tier-and-package-prices.php (version 1.1.16)
- composer.json (version 1.1.16)
- CHANGELOG.md (v1.1.16 section)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

includes/class-wc-tpp-admin.php

@@ -10,7 +10,17 @@ if (!defined('ABSPATH')) {
 if (!class_exists('WC_TPP_Admin')) {
     class WC_TPP_Admin {
 
-    public function __construct() {
+    private static $instance = null;
+    private static $settings_instance = null;
+
+    public static function get_instance() {
+        if (null === self::$instance) {
+            self::$instance = new self();
+        }
+        return self::$instance;
+    }
+
+    private function __construct() {
         add_filter('woocommerce_get_settings_pages', array($this, 'add_settings_page'));
         add_action('admin_enqueue_scripts', array($this, 'enqueue_admin_scripts'));
     }
@@ -19,7 +29,10 @@ if (!class_exists('WC_TPP_Admin')) {
      * Add settings page to WooCommerce settings
      */
     public function add_settings_page($settings) {
-        $settings[] = include WC_TPP_PLUGIN_DIR . 'includes/class-wc-tpp-settings.php';
+        if (null === self::$settings_instance) {
+            self::$settings_instance = include WC_TPP_PLUGIN_DIR . 'includes/class-wc-tpp-settings.php';
+        }
+        $settings[] = self::$settings_instance;
         return $settings;
     }
 
@@ -31,5 +44,5 @@ if (!class_exists('WC_TPP_Admin')) {
     }
 }
 
-new WC_TPP_Admin();
+WC_TPP_Admin::get_instance();
 }