Make a WooCommerce integration compatible with High-Performance Order Storage (HPOS)

Steps

1. Declare HPOS compatibility in your plugin by calling `\Automattic\WooCommerce\Utilities\FeaturesUtil::declare_compatibility('custom_order_tables', __FILE__, true)` inside the `before_woocommerce_init` action hook.
2. Audit all code that reads or writes order data: replace direct `$wpdb` queries on `wp_posts` and `wp_postmeta` with WooCommerce CRUD methods (`get_meta`, `update_meta_data`, `save`) on `WC_Order` objects.
3. Replace any code using `get_post_meta` / `update_post_meta` with order-specific equivalents: `$order->get_meta()` and `$order->update_meta_data()` followed by `$order->save()`.
4. Test the integration on a staging site with HPOS enabled (WooCommerce > Settings > Advanced > Features) and compatibility mode temporarily active to catch issues during migration.
5. Disable compatibility mode (which syncs data between old posts table and new HPOS tables) only after verifying the integration works correctly with HPOS-only storage.
6. If third-party plugins in the stack have not declared HPOS compatibility, WooCommerce will prevent enabling HPOS; coordinate upgrades across all plugins before switching production stores.