core: implement guarantee deposit locking in creditline

[EmeditWeb]

Problem

The parameters contract requires a minimum guarantee
of 20% of the loan amount. But create_loan() never
actually verifies or locks this guarantee deposit.

What To Build

1. Verify guarantee in create_loan()
2. Add guarantee_amount to Loan struct
3. Return guarantee on repayment
4. Forward guarantee to pool on default

Files To Touch

• contracts/creditline-contract/src/lib.rs
• contracts/creditline-contract/src/types.rs
• contracts/creditline-contract/src/tests.rs

Acceptance Criteria

• create_loan() verifies guarantee amount
• Insufficient guarantee rejected with typed error
• Guarantee returned on repayment
• Guarantee forwarded to pool on default
• All existing tests updated and passing

Mandatory Checks Before PR

• cargo build passes
• All tests pass
• PR references this issue

[mofejo1]

kindly assign this to me