BizCore v6.3 - Mundo Checkout & Seller Courier Control

SAFE additive update.
- Pending checkout orders reserve stock for 48 hours to reduce overselling.
- Cancelled pending orders release reservations.
- Confirmed orders consume reservations and use the existing BizCore stock deduction/invoice flow.
- Seller courier handling defaults to Platform Managed.
- Admin may enable Seller Managed per vendor.
- Seller-managed vendors can enter courier/tracking and mark orders shipped.
- Customer tracking API exposes seller shipment tracking.

Install: extract to the BizCore web root (overwrite), then open /update-6.3.php as Super Admin.
