Directional reciprocity bounds for null communication channels in weak gravitational fields

Null communication through a weak gravitational field can acquire a propagation-time shift. The usual scalar part of this shift is well known from Shapiro delay, but it does not by itself determine the directional dependence of the channel. This paper studies the difference between the first-order time shifts in two opposite communication directions between the same endpoints. We show that static scalar weak-field perturbations give equal one-way shifts at first order. Thus, static scalar curvature can delay the signal, but it does not generate first-order directional non-reciprocity. In contrast, a stationary vector metric component produces an antisymmetric contribution that changes sign when the propagation direction is reversed. We also derive a source-current bound that relates this directional term to the source momentum density, the channel length, and the source-channel separation. The analytical results are checked by direct null-geodesic propagation in a prescribed weak stationary rotating-source metric, followed by profile-based and source-current consistency tests. The same classification is placed in context using Shapiro delay, the Sagnac effect, terrestrial frame dragging, VLBI light-deflection scales, and representative rotating-source estimates. The static assumption is essential, and explicitly time-dependent scalar perturbations are outside this reciprocity statement. Within the stationary weak-field setting considered here, the results show that first-order directional non-reciprocity is not produced by static scalar curvature, but by vector, rotational, or gravitomagnetic structure whose magnitude is bounded by source momentum and channel geometry.