Mild UV–X-Ray Slope Evolution Collapses Apparent Extreme High-Redshift Distance Compression in Unified-Flow Quasar Cosmography

A joint analysis of Pantheon+SH0ES Type~Ia supernovae and a 2421-object quasar UV--X-ray catalog is presented within the Unified-Flow distance-scaling framework. The core hypothesis tested is that a mild, monotonic redshift evolution in the quasar UV--X-ray slope can absorb the apparent high-redshift residual drift that otherwise drives an extreme Unified-Flow distance-law transition. The Unified-Flow specification provides a mapping between a redshift-dependent scaling field and the luminosity-distance integral, while the quasar likelihood constrains the same distance through a non-linear UV--X-ray flux relation. Four nested model classes are compared: single- and two-regime distance laws, each combined with either a constant quasar UV--X-ray slope or a minimal redshift evolution of that slope. When the quasar slope is forced constant, the best fit drives the high-redshift distance-law exponent to an extreme value and yields strong compression of inferred distances above redshift unity. Allowing mild slope evolution removes the high-redshift residual drift that motivates the extreme branch and collapses the preferred distance-law transition to an order-unity regime. A leakage-controlled low-redshift calibration and multiple cross-validation schemes support the interpretation that an apparent extreme high-redshift distance transition is not robust to admitting small, monotonic evolution in quasar standardization. Quantitative predictions are stated for independent quasar catalogs and future high-redshift distance indicators.