Constant REVERSE_FROM 

const REVERSE_FROM: usize = 8;

The largest ladder the eight-lane engine still builds the Jacobian for.

Set from the table above: one eight-lane pass wins at four and eight pillars (1.2× and 1.3×), two passes lose at sixteen (1.1×), so the threshold sits at the one-pass width. Measured on the public lift path on an Apple M4 Pro; moving it is a one-line change argued against the jacobian_ladder_* groups in benches/mode_crossover.rs.

The two engines agree to a few ulp, not exactly: JetK and the reverse sweep accumulate a quotient’s tangent in different orders. The curve a calibration returns is bit-identical in every mode; only the last bits of its provenance Jacobian depend on which side of this threshold the ladder fell.