BizCore v5.9 Purchasing Core SAFE

Additive purchasing update. No existing business records are deleted or reset.

Includes:
- Draft / approved Purchase Orders with secure approval and cancellation.
- PO total discount/tax and supplier credit-day handling.
- Partial GRN validation: cannot receive more than PO remaining quantity.
- GRN now records subtotal, discount, tax, paid amount, balance, due date and supplier payable status.
- Stock IN and batch/expiry receiving retained.
- Direct Purchase protected by current-user password confirmation.
- Supplier payments protected by current-user password confirmation.
- Purchase Returns with returned-quantity tracking, stock OUT, batch reduction and supplier payable credit.
- Purchase Returns list and GRN return action.
- v5.6 database hardening, v5.6.2 supplier compatibility, v5.7 inventory inline masters and v5.8 step-up security compatibility retained in this patch where relevant.

Install:
1. Upload ZIP to the BizCore ceylongemx.online document root.
2. Extract/overwrite.
3. Open https://ceylongemx.online/update-5.9.php as Super Admin.
4. Run a test PO -> GRN -> Supplier Payment -> Purchase Return.
