Flashloan deactivation lacks a zero-debt invariant; residual raw-debt dust accumulates to `BorrowLimitReached` DoS

Flashloan borrow rounds raw debt up (`safe_div_ceil`) while payback rounds down (`safe_div`), and `flashloan_payback` always deactivates without a zero-debt invariant, so each exact cycle leaves about 1 raw unit of residual debt on the shared per-mint position; permissionless repetition accumulates it until `BorrowLimitReached` blocks all further flashloan borrows for that mint — a liveness DoS with no funds lost.