Hot column contaminating the overscan region

[rongpu]

As mentioned in this slack message, since a readout parameter change on 20250114 there is a degradation of a hot column in z1, and it significantly affects the overscan region of amp C. This led to a number of fibers in petal 1 to be flagged as "LOWEFFTIME" (desihub/desisurveyops#274).

To illustrate, below are morning darks before and after the readout change. The overscan region is on the right side of the red line. We will try a software fix by discarding the contaminated rows in the overscan image. The right panel shows another hot column on the same CCD which may be insufficiently masked at the moment.