Function check_quotes 

fn check_quotes(
    read_back: &[f64],
    dq: &[Vec<f64>],
    rungs: &[Rung],
    tolerance: Tolerance,
) -> Result<(), CalibrationError>

The two things a quote block must satisfy before it becomes a ladder: every rung moves under its own pillar, and every rung reads back the quote it was declared at.

Both tests are written as negated passes so a NaN — a priced rung with no money per quote, an upfront rung on zero notional — fails them: a plain comparison let NaN through, and a ladder of NaN then shipped as Ok.