Variable metadata held by the Model. Users do not construct this
directly, they get an Expr back from VarBuilder::build and look up
solution values via crate::Model / oximo_solver::SolverResult.
Display name of v within vars, degrading to variable #<index> when the
id is out of range (a foreign or not-yet-registered VarId). Used to build
human-readable error messages that name the offending variable.