core: implement loan default detection and liquidation

[EmeditWeb]

Problem

There is no mechanism to detect or process loan
defaults. If a learner stops paying, the loan
stays Active forever. Sponsors never recover their
locked funds. The protocol has no loss management.

What To Build

1. Add check_default(env, loan_id) to creditline
2. Add can_default(env, loan_id) view function
3. Add LOANDEFAULTED event to events.rs
4. Update liquidity pool liquidate_funds()
5. Write comprehensive tests

Files To Touch

• contracts/creditline-contract/src/lib.rs
• contracts/creditline-contract/src/events.rs
• contracts/creditline-contract/src/errors.rs
• contracts/liquidity-pool-contract/src/lib.rs
• contracts/creditline-contract/src/tests.rs

Acceptance Criteria

• check_default() correctly detects defaults
• Defaulted loan cannot be paid or defaulted again
• Pool funds correctly liquidated
• Reputation penalized -30 on default
• All existing tests still pass

Mandatory Checks Before PR

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