Annotate literals in Constrained generators

When you generate a value for a variable and substitute you risk losing information about _what_ variable caused a later constraint to be unsat. It would be good to have constraints be labeled by what variables went into them to make it easier to identify what made a constraint unsat.

The issue does not name a file, test, or entry point. Start by locating the constrained-generator substitution path and determine how generated literals become constraints. Done means constraints retain labels identifying the variables that contributed to them, with tests demonstrating that information during an unsatisfiable result.

Written by the indexing model from the issue text.