rugnux --mode scale: hold the merge cell by value, not a reference into a temporary

GetUnitCell() returns std::optional by value, so a reference to its value() dangled at the end of
the statement (clang -Wdangling-gsl).

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01D1G8gJVAy6gp1K5Dz3NE5C

rugnux/rugnux_cli.cpp

@@ -1634,7 +1634,7 @@ static int RunRugnux(int argc, char **argv) {
         // each frame's own goniometer rotation. Say so here, where both cells can be named, instead of
         // merging indices in a frame they do not belong to.
         {
-            const auto &merge_cell = experiment.GetUnitCell().value();
+            const UnitCell merge_cell = experiment.GetUnitCell().value();
             const float dist_tol = experiment.GetIndexingSettings().GetUnitCellDistTolerance();
             const float ang_tol = experiment.GetIndexingSettings().GetUnitCellAngleTolerance_deg();
             size_t n_indexed = 0, n_agree = 0;
@@ -1677,7 +1677,7 @@ static int RunRugnux(int argc, char **argv) {
         // the observations in another. Nothing downstream can notice, so the check belongs here, with
         // both the cell and the group in hand.
         if (const auto merge_sg = experiment.GetGemmiSpaceGroup()) {
-            const auto &uc = experiment.GetUnitCell().value();
+            const UnitCell uc = experiment.GetUnitCell().value();
             const double violation = MetricViolation(uc, *merge_sg);
             if (violation > MAX_METRIC_VIOLATION) {
                 logger.Error("The space group {} cannot describe this cell: a={:.3f} b={:.3f} c={:.3f} "