track unusable fibers that aren't broken fibers or stuck positioners

Add a category of positioners that are moveable but shouldn't be used for science targets, sky fibers, or standard stars, e.g. because the fiber lands on a hot column of the CCD and won't produce a useable spectrum.

These are different from broken fibers, because they can be back illuminated and safely moved to arbitrary positions; we just don't want to count that in the bookkeeping as a good assignment for a science/stdstar/sky.

These are different from stuck positioners, because they are moveable, e.g. to avoid collisions.

The implementation might be a mix of desimodel/data/focalplane/ and fiberassign code. The main point is to just assign them somewhere that doesn't collide with a good positioner, while not have it count against the NUMOBS bookkeeping or SKY/STDSTAR budget.

Start by examining desimodel/data/focalplane/ and the fiberassign code to understand how broken fibers, stuck positioners, assignments, NUMOBS, and SKY/STDSTAR budgets are represented. Done means movable but unusable positioners are assigned collision-free locations without counting as valid science, sky, or standard-star assignments.

Written by the indexing model from the issue text.