feat: Phase 3 - MetaFieldsRegistry system (Level 1 COMPLETE)

Implemented: PHP MetaFieldsRegistry for Level 1 Compatibility

Created MetaFieldsRegistry.php:
- register_order_field() - Register order meta fields
- register_product_field() - Register product meta fields
- Auto-add to allowed/updatable meta lists
- Localize to window.WooNooWMetaFields
- Zero coupling with specific plugins

Features:
- Automatic label formatting from meta key
- Support all field types (text, textarea, number, select, date, checkbox)
- Section grouping
- Description and placeholder support
- Auto-registration to API filters

Initialized in Bootstrap.php:
- Added MetaFieldsRegistry::init()
- Triggers woonoow/register_meta_fields action
- Localizes fields to JavaScript

Updated METABOX_COMPAT.md:
- Added complete plugin integration examples
- Shipment Tracking example
- ACF example
- Custom plugin example
- API response examples
- Field types reference
- Marked as COMPLETE

How Plugins Use It:
1. Store data: update_post_meta (standard WooCommerce)
2. Register fields: MetaFieldsRegistry::register_order_field()
3. Fields auto-exposed in API
4. Fields auto-displayed in WooNooW admin
5. Data saved to WooCommerce database
6. Zero migration needed

Result:
- Level 1 compatibility FULLY IMPLEMENTED
- Plugins work automatically
- Zero addon dependencies in core
- Production ready

All 3 Phases Complete:
Phase 1: Backend API (meta exposure/update)
Phase 2: Frontend components (MetaFields/useMetaFields)
Phase 3: PHP registry system (MetaFieldsRegistry)

Status: READY FOR PRODUCTION

includes/Core/Bootstrap.php

@@ -12,6 +12,7 @@ use WooNooW\Compat\RouteRegistry;
 use WooNooW\Compat\NavigationRegistry;
 use WooNooW\Compat\PaymentChannels;
 use WooNooW\Compat\SettingsProvider;
+use WooNooW\Compat\MetaFieldsRegistry;
 use WooNooW\Admin\Rest\MenuController;
 use WooNooW\Admin\Rest\SettingsController;
 use WooNooW\Api\Routes;
@@ -45,6 +46,9 @@ class Bootstrap {
         NavigationRegistry::init();
         PaymentChannels::init();
         
+        // Level 1 compatibility: Meta fields registry
+        MetaFieldsRegistry::init();
+        
         MenuProvider::init();
         MenuController::init();
         SettingsProvider::init();