xls-66: fix equation (23) to include managementFee_overpayment

[Tapanito]

High Level Overview of Change

Problem

Equation (23) stated:

principalPortion = overpaymentAmount - overpaymentInterest_net - overpaymentFee

This is incorrect. overpaymentInterest_net is the vault's share of the overpayment interest — the management fee has been netted out of it (formula 21), but the management fee is still a real cash outflow that leaves the overpayment amount and goes to the broker. The original formula effectively leaked the management fee into the principal reduction, overstating it.

The accompanying note ("The management fee has already been deducted from overpaymentInterest_net, so it does not appear as a separate term") compounded the error by providing a justification for the wrong formula.

Fix

The correct money conservation identity is:

overpaymentAmount = principalPortion + overpaymentInterest_net + managementFee_overpayment + overpaymentFee

Substituting formula (21) confirms this reduces to principalPortion = overpaymentAmount - overpaymentInterest_gross - overpaymentFee — no double subtraction. Equation (23) is updated accordingly and the incorrect note is removed.

This is already consistent with Appendix A-3.2.3 and the rippled implementation in LendingHelpers.cpp:

.trackedPrincipalDelta = overpayment - roundedOverpaymentInterest
                       - roundedOverpaymentManagementFee
                       - overpaymentFee,

Context of Change

Type of Change

• New XLS Draft
• XLS Update (changes to an existing XLS)
• XLS Status Change (e.g., Draft → Final, Draft → Stagnant)
• Process/Meta (changes to CONTRIBUTING.md, XLS-1, templates, etc.)
• Infrastructure (CI, workflows, scripts, website)
• Documentation (README updates, typo fixes)