A dark matter halo supported by internal pressure makes a two-sided prediction: at fixed mass its core grows with the halo velocity scale (expansion), and at fixed velocity its core shrinks as the central baryonic surface density rises (compression). All previous evidence for this came from rotation-curve fits of the pressure model itself, leaving open the objection that the pattern is a fitting artifact. Here we test both signs in core measurements involving no such fit: 55 galaxies from three published samples, measured by three independent teams using three independent techniques, H i density profiles, stellar kinematics, and Hα velocity fields. The analysis is pre-registered: predictions, sign conventions, statistics, and verdict criteria were fixed before the data were examined, with permutation-based significance and the code deposited publicly. The expansion sign appears at partial r = +0.53 (p = 0.007, N = 26). The compression sign, the load-bearing prediction, is aligned in all three samples and reaches a combined one-sided p = 0.031 (N = 55) under a rule in which a reversed sign would have counted against the model; this value is marginal under sequential correction and does not survive removal of the strongest sample (p = 0.174). Of six tests, five carry the predicted sign, one is null, none is reversed. A parameter-free Jeans ratio holds with 0.197 dex scatter. These results establish the pattern at first-sighting strength, not the mechanism: it remains degenerate with feedback-driven core formation, and the discriminating simulation test is the sequel.
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