Does the Late-Time Universe Demand Λ When the Distance Ruler Cannot Curve?

Recent analyses have demonstrated that Type Ia supernova (SN Ia) distance residuals contain ordered structure that survives aggressive control of calibration and selection effects. A persistent concern is that such structure may arise from curvature freedom inadvertently introduced through redshift-dependent correction operators rather than from physical cosmology. In this work, I impose an explicit fixed-ruler constraint: all redshift-polynomial curvature components are stripped from the SN correction operator before cosmological inference. Using Pantheon+ supernovae jointly with BAO measurements from DESI, I compare flat ΛCDM against Λ = 0 curved FRW models under both fixed and curvature-allowed ruler constructions. I find that Λ is strongly preferred in both cases, with ∆χ2 ≃ 60 against Λ = 0 even when the ruler is forbidden from curving. This demonstrates that the empirical demand for a cosmological constant is not an artifact of correction-induced curvature freedom. These results constrain the physical origin of SN residual structure without invoking expansion-history freedom.