BizCore v5.2 - Mundo Marketplace Commerce

Safe incremental update. Install v5.0.3 and v5.1 first.

Adds:
- Vendor product listing / approval management
- Product-specific or inherited vendor commission rules
- Vendor commission allocation when an online order is confirmed
- Vendor earnings / unsettled balance reporting foundation
- Payment gateway and courier assignment on Mundo orders
- Tracking number and shipment ledger foundation
- Marketplace order API vendor/gateway/courier linkage

Install:
1. Extract into the BizCore web root and overwrite included files.
2. Open /update-5.2.php as Super Admin.
3. Click Install Mundo v5.2.

No existing BizCore sales, stock, finance, production, contacts, users or Mundo orders are deleted/reset.
