Release version 1.1.18 - Fix root cause of duplicate settings

Identified and fixed the actual root cause of duplicate settings page:
automatic instantiation in settings file being executed multiple times.

Root Cause Analysis:
- Settings file ended with: return new WC_TPP_Settings();
- File is in Composer's classmap for autoloading
- When autoloader loads class, it executes the entire file including instantiation
- Each include/autoload created a NEW instance despite admin singleton pattern
- Admin singleton prevented multiple admin instances but not multiple settings instances

The Fix:
- Removed automatic instantiation (return new) from settings file
- Settings file now only contains class definition
- Admin class explicitly creates instance: new WC_TPP_Settings()
- Changed from include to require_once to prevent multiple file loads
- Settings instance now created exactly once, when filter needs it

Fixes:
- Settings page rendering twice in WooCommerce backend
- Multiple WC_TPP_Settings instances being created
- Composer autoload triggering unintended instantiation

Changes:
- Removed line 145 from class-wc-tpp-settings.php (return new WC_TPP_Settings())
- Modified add_settings_page() to use require_once + explicit new
- Settings file now side-effect free, safe for autoloading

Technical Details:
- Composer classmap autoloader includes files when class is referenced
- Previous code had side effect (instantiation) on every include
- New code separates class definition from instantiation
- Instance creation now controlled explicitly by admin class
- require_once ensures file loaded once even if accessed multiple times

Updated Files:
- includes/class-wc-tpp-settings.php (removed return new line)
- includes/class-wc-tpp-admin.php (explicit instantiation)
- wc-tier-and-package-prices.php (version 1.1.18)
- composer.json (version 1.1.18)
- CHANGELOG.md (v1.1.18 section)

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

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

includes/class-wc-tpp-settings.php

@@ -141,5 +141,3 @@ if (!class_exists('WC_TPP_Settings')) {
     }
 }
 }
-
-return new WC_TPP_Settings();