core: implement principal-interest-fee repayment waterfall in creditline

[EmeditWeb]

Problem

The Loan struct (types.rs:63-72) tracks separate outstanding buckets —
principal_outstanding, interest_outstanding, service_fee_outstanding
and late_fees_outstanding — but repay_installment() (lib.rs:755-813)
only subtracts the paid amount from remaining_balance. The individual
buckets are never decremented, so after any installment payment the loan's
internal accounting is inconsistent: interest earned by LPs, service-fee
revenue, and default-penalty calculations (which read these buckets) all
operate on stale values.

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

A deterministic repayment waterfall applied on every repayment
(repay_installment and repay_loan):

1. Allocate the paid amount in fixed priority order:
   late fees → accrued interest → service fee → principal.
2. Decrement each *_outstanding bucket accordingly and only report the
   principal/interest split (per bucket) to the pool.
3. Reject payments that would underflow any bucket; use safe_math.
4. Keep remaining_balance equal to the sum of all outstanding buckets as
   an invariant, asserted in tests.

Files To Touch

• contracts/creditline-contract/src/lib.rs (repayment functions, helper)
• contracts/creditline-contract/src/safe_math.rs (if new helpers needed)
• contracts/creditline-contract/src/tests.rs

Acceptance Criteria

• Waterfall order enforced: late fee → interest → service fee → principal
• Each *_outstanding bucket decremented correctly per payment
• remaining_balance == sum(all outstanding buckets) invariant holds
• Interest/principal split reported to pool matches allocation
• Partial and full payments both covered by tests

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

[Danielodingz]

Hey maintainer, please i would like to work on this issue.