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/test/main.cpp

@@ -175,6 +175,7 @@ int main(int argc, char ** argv) {
     TST(var_subst);
     TST(simple_parser);
     TST(api);
+    TST(bnh_opt);
     TST(api_algebraic);
     TST(api_polynomial);
     TST(api_pb);