Metaobject definition references export by handle

Metaobject definitions that reference other metaobjects can now be exported from one store and imported into another.

🚀 What’s New

• Cross-store metaobject definitions – field validations that reference other metaobject definitions now use portable type handles instead of store-specific GIDs.
• Flexible import format – imports accept both comma-separated type handles (author, brand) and legacy GID format for backwards compatibility.

🔧 Improvements

• Cleaner exports – multiple definition references now export as author, brand, store_location instead of JSON arrays.

🐛 Fixes

• Reference validation – importing a definition that references a non-existent metaobject definition now shows a clear error message (MOBD006) instead of failing silently.