Unify handling of unassigned values when unassigned not allowed

We recently introduced a structural score to properly define what happens to entity loops.
This means we now have a mechanism to unify what happens to solutions with unassigned values, when unassigned are not allowed.
Here's the proposal:

• Unassigned value count becomes part of structural score, if unassigned is not allowed. (If unassigned is allowed, then a solution with unassigned values is perfectly valid.)
• Solver fails fast if such a solution is submitted, but CH is not configured.
• Score analysis accepts such a solution, and we enhance the structural analysis report to include this information.

This should be consistent with how we handle entity loops.
If any of this should not be possible due to backwards compatibility, let's discuss.

Start by locating the existing structural score and entity-loop handling, then trace how unassigned values are processed by the solver and score analysis. Review the structural analysis report and CH configuration paths. Done means the three proposed behaviors are implemented consistently, with coverage for allowed and disallowed unassigned values.