Seller financing as a secondary loan (% of equity)

[georgymh]

Allow part of the equity component to be sourced from a seller-financed second loan.

Open questions

• Input model: fraction of equity (%) financed by seller, or absolute $? Both? Default to % to mirror the down-payment input style.
• Independent rate / term (and IO period — see Seller financing as a secondary loan (% of equity) #3) for the seller note.
• Sources & Uses impact: Loan Proceeds becomes Senior + Seller; Equity shrinks accordingly.
• DSCR: include seller-note debt service in the same ratio, or report "senior DSCR" and "all-in DSCR"?
• Lender concerns (CLTV cap, subordination) are out of scope for the calculator, but consider a hint or warning.
• Does it change the "Cash Invested" definition used for CoC?