viewer: calibrate a sample cell against its own space group, as the CLI does

The powder-calibration widget built its ring list with CalculateXtalRings(cell),
which assumes a primitive lattice. The overload taking a space group exists
precisely because the fit pairs the innermost OBSERVED ring with the innermost
LISTED one, so a list opening with a reflection the symmetry forbids scales the
whole calibration by the ratio between that ring and the first real one.

rugnux_cli was moved to the new overload when it landed; the viewer was not. So
on any centred sample cell the viewer's calibration was scaled wrong - the exact
failure the overload was added to prevent.

The widget already takes the cell from the loaded dataset; it now takes the group
with it and uses both. Where the dataset has no group the old call stands, which
is the same P-lattice assumption as before and no worse.

Found by a whole-branch review, which saw the two call sites side by side where a
per-commit read of either could not.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

viewer/widgets/PowderCalibrationWidget.cpp

@@ -80,8 +80,15 @@ std::vector<float> PowderCalibrationWidget::GetCalibrantRings() const {
     const auto &table = Calibrants();
     if (idx >= 0 && idx < static_cast<int>(table.size()))
         return CalibrantRings(table[idx].name);
-    if (sample_cell)
+    if (sample_cell) {
+        // With the group where the dataset has one: the absences then come from the symmetry rather
+        // than from a bare P-lattice assumption, and a centred cell's list no longer opens with a
+        // reflection that is not there - which would scale the whole calibration by the ratio between
+        // that ring and the first real one. Same call the CLI makes.
+        if (sample_sg)
+            return CalculateXtalRings(sample_cell.value(), *sample_sg);
         return CalculateXtalRings(sample_cell.value());
+    }
     return CalibrantRings(table.front().name);
 }
 
@@ -108,8 +115,10 @@ void PowderCalibrationWidget::updateCalibrantList() {
 }
 
 void PowderCalibrationWidget::loadImage(std::shared_ptr<const JFJochReaderImage> image) {
-    if (image)
+    if (image) {
         sample_cell = image->Dataset().experiment.GetUnitCell();
+        sample_sg = image->Dataset().experiment.GetGemmiSpaceGroup();
+    }
     updateCalibrantList();
 }
 

viewer/widgets/PowderCalibrationWidget.h

@@ -4,6 +4,7 @@
 #pragma once
 
 #include <QWidget>
+#include <gemmi/symmetry.hpp>
 #include <QCheckBox>
 #include <QComboBox>
 #include <QVector>
@@ -26,6 +27,9 @@ struct CalibrationSelection {
 class PowderCalibrationWidget : public QWidget {
     Q_OBJECT
     std::optional<UnitCell> sample_cell;
+    // Taken with the cell, and used with it: the ring list a user-supplied cell produces has to carry
+    // the group's absences, or the fit pairs the innermost OBSERVED ring with a forbidden one.
+    std::optional<gemmi::SpaceGroup> sample_sg;
 
     QComboBox* calibrantCombo{nullptr}; // stores current calibrant selection
     QComboBox* methodCombo{nullptr};    // rings / spots, for the whole-dataset run