ProductionFactorSegment struct
One segment of the piecewise-linear efficiency curve.
Each segment contributes constant + slope × (V − volume) to the concave envelope. The overall efficiency at volume V is the minimum over all segments (matching PLP Fortran FRendimientos).
Note: Unlike ReservoirSeepageSegment where constant is the y-intercept at V = 0, here constant is the efficiency value at the breakpoint volume (point-slope form).