don't assign stdstar and sky next to each other on CCD

We shouldn't assign standard stars and sky locations to fibers that are neighboring *on the CCD* due to the possibility of the bright standard star contaminating the neighboring sky due to unmodeled spectrograph PSF wings. Currently fiber assignment only knows about the locations of positioners *on the focal plane*, but doesn't know where those fibers go in the spectrograph CCDs. It is quite possible for fibers that are not neighbors on the focal plane to be neighbors on the CCD.

desimodel/data/focalplane/fiberpos.* contains the mapping (to be updated after the real focal plane is assembled) between the fiber number and the focal plane location. Fiber assign should read this file and use that information to avoid assigning sky and standards next to each other.

To repeat: this is about not assigning sky and stds next to each other *on the CCD*, not about their relative locations on the focal plane.

Start by locating the fiber-assignment code that currently uses focal-plane positioner locations, then inspect desimodel/data/focalplane/fiberpos.* for the fiber-to-location mapping. Determine how CCD-neighbor relationships should be represented and use them to prevent neighboring sky and standard assignments; completion means the assignment logic distinguishes CCD adjacency from focal-plane adjacency and handles the updated mapping.

Written by the indexing model from the issue text.