Full text index descriptors/range incompatibility

When implementing `Range.KeyRangeLookup`, some of the tests panicked because `len(Range.Fields) != Range.Desc.Count()`. I worked around the problem but did not dig into why this is the case. It seems suspicious and could cause issues elsewhere.

Locate the tests that exercise Range.KeyRangeLookup and the definitions of Range.Fields and Range.Desc.Count(). Run those tests to reproduce the panic. Done means the descriptor and field incompatibility is understood and the relevant tests no longer panic.

Written by the indexing model from the issue text.