Structural Non-Closure in Galactic Rotation Curves: A Regime-Dependent Diagnostic Using SPARC Data

Galactic rotation curves remain one of the most persistent empirical challenges in modern astrophysics. Most explanatory frameworks — including dark matter halo models and modified gravity proposals — implicitly assume that a single dynamical law or universal correction should account for the kinematic behavior of spiral galaxies across heterogeneous observational regimes. This technical note explores a different diagnostic perspective. Instead of proposing a new gravitational theory, the analysis examines whether the expectation of global explanatory closure itself may be structurally fragile when galaxies with heterogeneous observational traces are evaluated under a single correction framework. Using the SPARC (Spitzer Photometry and Accurate Rotation Curves) dataset as a diagnostic reference sample, a minimal long-range correction proxy is introduced in order to test the stability of a single global correction scale across galaxies with different structural properties and observational depth. The analysis suggests that improvements obtained under a fixed global correction scale tend to organize according to structural regime: extended spiral systems may exhibit coherent adjustments under the diagnostic proxy, while smaller or observationally limited systems remain inconsistent under the same global parameterization. A second structural aspect concerns the comparability of observational traces. Galaxies in the SPARC dataset differ substantially in radial coverage, sampling density, surface brightness, and signal-to-noise characteristics. These differences affect the informational depth of rotation-curve measurements and complicate the legitimacy of direct cross-system comparison under a single global fitting criterion. Filtering strategies based on trace robustness can stabilize inferred parameters across subsets of the dataset but do not restore universal closure across the full sample. Taken together, these observations suggest that the rotation-curve problem may involve two interacting structural layers: (1) the legitimacy of cross-system trace comparability and (2) the amplitude of regime-dependent corrections. The purpose of this work is methodological rather than theoretical. The correction proxy used in the analysis carries no ontological commitment and is intended solely as a diagnostic tool to illustrate how structural limits of global closure may arise when heterogeneous observational regimes are forced into a single universal correction framework.