Observer-Vertex Compatibility of Dressed Timescape Observables: A Low-Redshift Obstruction in a General-Spacetime Density Diagnostic

Koksbang and Heinesen recently introduced a general-spacetime quantity, 𝓜(z), constructed from the angular-diameter distance d_A(z) and the line-of-sight expansion rate 𝓗(z). We apply it to the standard dressed tracker observables of timescape cosmology and find a low-redshift compatibility obstruction that is not visible when the calculation is restricted to z≥0.38. General observer-centred cosmography requires a regular spacetime pair d_A(z),𝓗(z) to share one deceleration coefficient at the observer. The published timescape dressed H(z) and d_A(z) agree in the leading Hubble term but give different second-order coefficients. Writing q_H for the coefficient inferred from H and q_d for that inferred from distance, the tracker gives Δq≡q_H−q_d=f_v0(4f_v0−1)/(4f_v0²+f_v0+4). Consequently 𝓜_TS/H₀²=−(2/3)Δq/z+O(1). For f_v0=0.763, Δq=0.22078, the coefficient is −0.14718/z, and the exact curve crosses zero at z=0.20584. We interpret this as an observer-vertex applicability obstruction to treating the standard dressed H and d_A simultaneously as observables of one regular Lorentzian spacetime in the Koksbang–Heinesen identity, not as a null-energy-condition violation. At finite redshift the previously derived closed tracker expression remains non-constant and its positive branch above z=0.38 retains a turnover near z=0.777. Independently, we derive an exact interval-integrated form of 𝓜. An audit-corrected DESI DR2–Pantheon+ mock shows that the tested interval reconstructions are too ill-conditioned for a useful current-data measurement.