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

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(() => {