Function band 

fn band(scheme: Interpolation, h: f64) -> (f64, f64)

The two band coefficients of the continuity system on a segment of length h: the off-diagonal A and the half-diagonal B.

For a cubic these are the familiar h/6 and h/3. The tension spline generalises them, and σ → 0 recovers the cubic exactly — which is why one system serves both families. Both depend on h and the tension alone, never on an ordinate: that is what keeps the solve a linear map and the adjoints exact.