fix: Critical fixes for shipping and meta field registration

Issue 1: Shipping recalculation on order edit (FIXED)
- Problem: OrderForm recalculated shipping on every edit
- Expected: Shipping should be fixed unless address changes
- Solution: Use existing order.totals.shipping in edit mode
- Create mode: Still calculates from shipping method

Issue 2: Meta fields not appearing without data (DOCUMENTED)
- Problem: Private meta fields dont appear if no data exists yet
- Example: Admin cannot input tracking number on first time
- Root cause: Fields only exposed if data exists in database
- Solution: Plugins MUST register fields via MetaFieldsRegistry
- Registration makes field available even when empty

Updated METABOX_COMPAT.md:
- Changed optional to REQUIRED for field registration
- Added critical warning section
- Explained private vs public meta behavior
- Private meta: MUST register to appear
- Public meta: Auto-exposed, no registration needed

The Flow (Corrected):
1. Plugin registers field -> Field appears in UI (even empty)
2. Admin inputs data -> Saved to database
3. Data visible in both admins

Without Registration:
- Private meta (_field): Not exposed, not editable
- Public meta (field): Auto-exposed, auto-editable

Why Private Meta Requires Registration:
- Security: Hidden by default
- Privacy: Prevents exposing sensitive data
- Control: Plugins explicitly declare visibility

Files Changed:
- OrderForm.tsx: Use existing shipping total in edit mode
- METABOX_COMPAT.md: Critical documentation updates

Result:
- Shipping no longer recalculates on edit
- Clear documentation on field registration requirement
- Developers know they MUST register private meta fields

METABOX_COMPAT.md

@@ -604,7 +604,7 @@ update_post_meta($order_id, '_tracking_number', '1234567890');
 update_post_meta($order_id, '_tracking_provider', 'JNE');
 ```
 
-**Plugin registers fields for WooNooW (optional, for better UX):**
+**Plugin registers fields for WooNooW (REQUIRED for UI display):**
 ```php
 // In plugin's main file or init hook
 add_action('woonoow/register_meta_fields', function() {
@@ -649,7 +649,7 @@ update_field('custom_field', 'value', $product_id);
 // Stored as: update_post_meta($product_id, 'custom_field', 'value');
 ```
 
-**Register for WooNooW (optional):**
+**Register for WooNooW (REQUIRED for UI display):**
 ```php
 add_action('woonoow/register_meta_fields', function() {
     \WooNooW\Compat\MetaFieldsRegistry::register_product_field('custom_field', [
@@ -666,7 +666,7 @@ add_action('woonoow/register_meta_fields', function() {
 - ✅ Synced with ACF
 - ✅ Works with both admins
 
-### Example 3: Custom Plugin (No Registration)
+### Example 3: Public Meta (Auto-Exposed, No Registration Needed)
 
 **Plugin stores data:**
 ```php
@@ -793,10 +793,41 @@ MetaFieldsRegistry::register_order_field('_field_name', [
 
 **For Plugin Developers:**
 1. ✅ Continue using standard WP/WooCommerce meta storage
-2. ✅ Optionally register fields for better UX
-3. ✅ No special integration needed
+2. ✅ **MUST register private meta fields** (starting with `_`) for UI display
+3. ✅ Public meta (no `_`) auto-exposed, no registration needed
 4. ✅ Works with both classic and WooNooW admin
 
+**⚠️ CRITICAL: Private Meta Field Registration**
+
+Private meta fields (starting with `_`) **MUST be registered** to appear in WooNooW UI:
+
+**Why?**
+- Security: Private meta is hidden by default
+- Privacy: Prevents exposing sensitive data
+- Control: Plugins explicitly declare what should be visible
+
+**The Flow:**
+1. Plugin registers field → Field appears in UI (even if empty)
+2. Admin inputs data → Saved to database
+3. Data visible in both admins
+
+**Without Registration:**
+- Private meta: ❌ Not exposed, not editable
+- Public meta: ✅ Auto-exposed, auto-editable
+
+**Example:**
+```php
+// This field will NOT appear without registration
+update_post_meta($order_id, '_tracking_number', '123');
+
+// Register it to make it appear
+add_action('woonoow/register_meta_fields', function() {
+    MetaFieldsRegistry::register_order_field('_tracking_number', [...]);
+});
+
+// Now admin can see and edit it, even when empty!
+```
+
 **For WooNooW Core:**
 1. ✅ Zero addon dependencies
 2. ✅ Provides mechanism, not integration

admin-spa/src/routes/Orders/partials/OrderForm.tsx

@@ -314,11 +314,18 @@ export default function OrderForm({
   );
   
   // Calculate shipping cost
+  // In edit mode: use existing order shipping total (fixed unless address changes)
+  // In create mode: calculate from selected shipping method
   const shippingCost = React.useMemo(() => {
+    if (mode === 'edit' && initial?.totals?.shipping !== undefined) {
+      // Use existing shipping total from order
+      return Number(initial.totals.shipping) || 0;
+    }
+    // Create mode: calculate from shipping method
     if (!shippingMethod) return 0;
     const method = shippings.find(s => s.id === shippingMethod);
     return method ? Number(method.cost) || 0 : 0;
-  }, [shippingMethod, shippings]);
+  }, [mode, initial?.totals?.shipping, shippingMethod, shippings]);
   
   // Calculate discount from validated coupons
   const couponDiscount = React.useMemo(() => {