Temperature profile for a single line of sight. Top: Total observed brightness temperature T(ν) (black) and the underlying HI signal along the same LOS (red). Bottom: Zoom on the frequency range containing the prominent peak (note the logarithmic T scale). Star symbols mark the channels flagged by our sigma-clipping criterion and replaced by the second-pass polynomial baseline (Appendix B). This example corresponds to the line of sight hosting the brightest HI source in the simulation and illustrates the masking procedure.