Unified Spacetime Fluid Cosmology III: Observational Identity of a Minimal Late-Time Spacetime Sector

We present a conservative observational characterization of a minimal late-time spacetime sector in Unified Spacetime Fluid (USF) cosmology. The goal of this paper is not to establish a unique microphysical origin, but to determine whether a coherent low-redshift signal, within a framework inheriting background and growth consistency from Papers I and II, survives tests of local coherence, regional replication, and robustness against competing astrophysical interpretations. The merged baseline sample contains 1820 objects, 1623 coordinate-valid objects, 6085 local pairs, and 57 detected peak components. These counts are used as bookkeeping and reproducibility anchors rather than as independent physical observables. The load-bearing evidence is instead the survival of the associated skeleton-level organization under cleaning, redetection, null ensembles, and standalone reruns. The primary region-z-preserving null ensemble places the observed recall-based skeleton-recovery statistic in the extreme tail of 500 null realizations, corresponding to p_null = 1/501 ≃ 0.001996 under the finite-ensemble convention. The results indicate that host/environment contamination is significant but not dominant. We therefore interpret the late-time behavior phenomenologically as effective phase-transition-like behavior, or an effective two-stage late-time transition, with localized excitation-like structure embedded in a connected coarse-grained envelope. This paper defines an observational identity and falsifiability framework for a minimal late-time USF sector without requiring an overstrong claim of established vacuum excitation.