core: settle installment repayments to liquidity pool in creditline

[EmeditWeb]

Problem

repay_installment() in creditline-contract (lib.rs:755-813) marks
installments paid and decrements remaining_balance, but it never moves
any tokens and never calls the liquidity pool. Compare repay_loan()
(lib.rs:701-704) which correctly calls
lp_client.receive_repayment(principal, interest) and transfers USDC.

As a result a borrower can drive a loan all the way to LoanStatus::Paid
through installments without any USDC ever leaving their wallet, and
LPs earn zero yield on the installment repayment path. This is a
critical accounting and value-loss bug in the primary repayment flow.

Before Starting

Read ALL of these before writing any code:

• context/architecture-context.md
• context/code-standards.md
• context/progress-tracker.md

What To Build

Make repay_installment() a real settlement operation, symmetric with
repay_loan():

1. Pull the installment amount in USDC from the borrower into the
   contract (token transfer with require_auth).
2. Split the paid amount into principal vs interest (see the
   repayment-waterfall issue) and forward it to the pool via
   LiquidityPoolContractClient::receive_repayment(principal, interest)
   so locked_liquidity is reduced and interest raises share price.
3. Only transition to LoanStatus::Paid once real funds have settled.
4. Keep all state effects before external calls (checks-effects-interactions)
   and inside the existing non-reentrant guard.

Files To Touch

• contracts/creditline-contract/src/lib.rs (repay_installment)
• contracts/creditline-contract/src/tests.rs

Acceptance Criteria

• repay_installment() transfers USDC from borrower to contract
• Interest portion routed to pool via receive_repayment
• Pool locked_liquidity decreases by the principal repaid
• Loan reaches Paid only after funds settle on-chain
• Reentrancy guard and CEI ordering preserved
• Tests assert token balances and pool share-price movement end to end

Mandatory Checks Before Opening PR

• All context/ files read and understood
• context/progress-tracker.md updated
• cargo build passes with zero errors
• cargo test passes — test count not decreased
• require_auth() first in every mutating function
• extend_ttl() after every persistent storage write
• No .unwrap()/.expect() in user-facing paths
• PR references this issue number

[ebubeb683-ship-it]

I have experience using React, I have looked through the project’s code, and I can submit a Pull Request (PR) with the fix within 2 days.