core: implement atomic loan funding from liquidity pool

[EmeditWeb]

Problem

When a loan is approved via approve_loan(), the creditline
contract does not actually pull funds from the liquidity
pool contract. The approval is purely a status change.
There is no atomic linkage between the pool and the loan.
Funds are never actually moved on-chain when a loan is
approved.

Context

In a real BNPL protocol the liquidity pool must lock
capital against approved loans. Without this the pool
balance and loan balances are completely disconnected.
A sponsor could withdraw all funds while loans are
active. This is a critical protocol correctness issue.

Before Starting

Read these context files:

• context/architecture-context.md
• context/code-standards.md
• contracts/creditline-contract/src/lib.rs
• contracts/liquidity-pool-contract/src/lib.rs

What To Build

1. Add cross-contract call in approve_loan():
   After transitioning status to Active,
   call liquidity_pool.lock_funds(loan_id, amount)
   This marks that amount as locked in the pool.

2. Add lock_funds(env, loan_id, amount) to liquidity pool:

   • require_auth() for creditline contract address
   • Verify available_liquidity >= amount
   • Deduct from available_liquidity
   • Add to locked_liquidity
   • Store mapping: loan_id -> locked_amount

3. Add release_funds(env, loan_id) to liquidity pool:
   Called when loan is fully repaid.
   Moves locked amount back to available plus interest.

4. Add liquidate_funds(env, loan_id) to liquidity pool:
   Called when loan defaults.
   Distributes locked funds according to loss policy.

5. Update creditline repay_installment():
   On final installment, call pool.release_funds()

6. All cross-contract calls must be atomic.
   If pool.lock_funds() fails, approve_loan() must revert.

Files To Touch

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

Acceptance Criteria

• approve_loan() atomically locks funds in pool
• available_liquidity decreases on approval
• locked_liquidity increases on approval
• Final repayment releases funds back to pool
• Default triggers liquidation
• All operations revert if any step fails
• require_auth() on all mutating functions
• extend_ttl() after all storage writes
• All existing 96 tests still pass
• Minimum 8 new tests covering full lifecycle

Mandatory Checks Before PR

• cargo build passes with zero errors
• All tests pass
• No .unwrap() in user-facing paths
• PR references this issue

[divicbold47]

Pls maintaner assign me this task