Fix NLA optimization regression and relax restore_x

- Relax restore_x() to handle backup/current size mismatches: when
  backup is shorter (new columns added), call
  move_non_basic_columns_to_bounds() to find a feasible solution.
- Fix 100x performance regression in nonlinear optimization: save LP
  optimum before check_nla and return it as bound regardless of NLA
  result, so opt_solver::check_bound() can validate via full re-solve
  with accumulated NLA lemmas.
- Refactor theory_lra::maximize() into three helpers: max_with_lp(),
  max_with_nl(), and max_result().
- Add mk_gt(theory_var, impq const&) overload for building blockers
  from saved LP optimum values.
- Add BNH multi-objective optimization test (7/7 sat in <1s vs 1/7
  in 30s before fix).
- Add restore_x test for backup size mismatch handling.

Fixes #8890

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/math/lp/lar_solver.cpp

@@ -467,6 +467,8 @@ namespace lp {
         return ret;
     }
 
+
+
     lp_status lar_solver::solve() {
         if (m_imp->m_status == lp_status::INFEASIBLE || m_imp->m_status == lp_status::CANCELLED)
             return m_imp->m_status;
@@ -2303,16 +2305,6 @@ namespace lp {
         return m_imp->m_constraints.add_term_constraint(j, m_imp->m_columns[j].term(), kind, rs);
     }
 
-    struct lar_solver::scoped_backup {
-        lar_solver& m_s;
-        scoped_backup(lar_solver& s) : m_s(s) {
-            m_s.get_core_solver().backup_x();
-        }
-        ~scoped_backup() {
-            m_s.get_core_solver().restore_x();
-        }
-    };
-   
     void lar_solver::update_column_type_and_bound_with_ub(unsigned j, lp::lconstraint_kind kind, const mpq& right_side, u_dependency* dep) {
         SASSERT(column_has_upper_bound(j));
         if (column_has_lower_bound(j)) {