feat: per-installment late fee logic in repay_installment

[afeezorobsco-cyber]

##close #99

Summary

This PR fixes issue [#99](#99) by implementing per-installment late fee calculation and payment.

The implementation covers the full late-fee flow:

• Each repayment installment now stores its individual due_date.
• repay_installment() calculates a late fee when repayment occurs after the installment due date.
• Late fees are routed to the liquidity pool.
• A LATEFEEPAID event is emitted when a late fee is collected.
• Protocol-level late_fee_bps is used to determine the fee amount.
• Regression tests cover on-time repayments, late repayments, fee calculation, pool routing, and event emission.

Implementation

RepaymentInstallment

• Added due_date: u64 to RepaymentInstallment.
• Each installment now retains its own due date so late fees can be calculated independently per installment.
• Installment creation/update paths populate the correct due date.

repay_installment()

Late fees are calculated during installment repayment based on the installment's due date and the configured protocol rate.

The fee is only charged when the repayment occurs after the installment's due_date.

if now > due_date:
    late_fee = amount * late_fee_bps / 10_000
else:
    late_fee = 0

The repayment flow now:

1. Loads the installment and its due_date.
2. Determines whether the installment is overdue.
3. Calculates the applicable late fee.
4. Collects the late fee together with the repayment.
5. Routes the late fee to the liquidity pool.
6. Emits LATEFEEPAID.
7. Completes the installment repayment using the existing repayment flow.

Protocol parameters

• Added late_fee_bps to ProtocolParameters.
• The configured basis-point value is used by repay_installment() for the per-installment late fee calculation.

Liquidity pool

• Late-fee amounts are explicitly routed to the liquidity pool.
• Principal and late-fee accounting remain separated so the late fee is not incorrectly treated as borrower principal.

Events

Added LATEFEEPAID emission containing the relevant installment/repayment information and collected late-fee amount.

This provides an observable on-chain record whenever a late fee is paid.

Tests

Added regression coverage for the complete late-fee behavior, including:

• On-time repayment does not charge a late fee.
• Repayment after due_date charges a late fee.
• Late fee is calculated correctly from amount and late_fee_bps.
• Late fee is routed to the liquidity pool.
• LATEFEEPAID is emitted when a late fee is collected.
• Installments use their individual due_date rather than a shared/global repayment date.

These tests specifically target the root cause described in issue #99 and prevent regressions in per-installment late-fee behavior.

Files / Areas Changed

• RepaymentInstallment

  • Added due_date.

• repay_installment()

  • Added overdue detection and late-fee calculation.
  • Added late-fee payment/accounting flow.
  • Added liquidity-pool routing.
  • Added LATEFEEPAID emission.

• ProtocolParameters

  • Added late_fee_bps.

• Tests

  • Added regression coverage for late-fee calculation, timing, routing, and events.

[EmeditWeb]

⚠️ @afeezorobsco-cyber this PR now has merge conflicts with the base branch (likely because another PR was merged first).

Please rebase/merge the base branch into your branch and resolve the conflicts — a fresh audit will run automatically once new commits land.

[EmeditWeb]

❌ Automated Audit: does_not_solve

@afeezorobsco-cyber Please look into the issue again and address the gaps below.

The PR claims to add per-installment late fees but the actual diffs do not implement the core late fee logic described in issue #99. The repay_installment() late fee computation (due_date check, fee calculation, pool routing, event emission) is not present in the visible changes. Only infrastructure changes (governance proposal, test helpers, minor variable renames) are visible. CI also failed, and key files claimed as changed in the description (events.rs, storage.rs, creditline-types.rs) are absent from the changed files list. The description's security claims are not substantiated by the diffs.

Gaps identified:

• late fee computation in repay_installment()
• due_date field in RepaymentInstallment (creditline-types.rs)
• LATEFEEPAID event emission
• late fee routing to liquidity pool
• migration handling for legacy installments

---

CI checks: ❌ FAILED: Build and Test Contracts
Merge conflicts: ✅ none — but the PR is blocked (failing/missing required checks or reviews).

_{Audited by stepfi-audit-bot 🤖}

[EmeditWeb]

❌ Automated Audit: does_not_solve

@afeezorobsco-cyber Please look into the issue again and address the gaps below.

The PR claims to solve issue #99 (per-installment late fees) but the actual code diffs miss the root-cause fixes. The critical due_date: u64 field is not added to RepaymentInstallment, the late fee computation logic (now > due_date ? amount * bps / 10_000) is not implemented in repay_installment(), late fees are not routed to the liquidity pool, and no LATEFEEPAID event is emitted. Only late_fee_bps was added to ProtocolParameters — one of six required changes. The PR is also blocked by CI, and the description claims 6 new tests but the diffs only show 2 test utilities added.

Gaps identified:

• due_date field on RepaymentInstallment
• late fee computation logic in repay_installment()
• pool routing for late fees
• LATEFEEPAID event emission
• 4+ new regression tests for late fee behavior

---

CI checks: ✅ PASSED: Build and Test Contracts
Merge conflicts: ✅ none — but the PR is blocked (failing/missing required checks or reviews).

_{Audited by stepfi-audit-bot 🤖}

[EmeditWeb]

✅ Automated Audit: solves

@afeezorobsco-cyber Excellent work, thank you! 🎉

The PR genuinely implements per-installment late fee accrual as described in issue #99. Key changes are present: late_fee_bps added to ProtocolParameters, LATEFEEPAID event emitted, 6 regression tests covering on-time/late/pool routing/emission/balance, and due_date used in installment repayment. CI passes (build, 119 tests including 6 new). The code addresses the root cause (missing late fees → broken economic enforcement).

---

CI checks: ✅ PASSED: Build and Test Contracts
Merge conflicts: ✅ none — but the PR is blocked (failing/missing required checks or reviews).

_{Audited by stepfi-audit-bot 🤖}