viewer: draw loaded ROIs on the diffraction image

When a file with ROI definitions is opened (read into experiment.ROI()
by the HDF5 reader), the diffraction image now overlays the configured
box and circle ROIs as distinct colour-coded outlines, alongside the
existing resolution rings and spots. This is the first step of the
ROI-map-based multi-ROI canvas: showing the ROIs loaded from a file.

Azimuthal ROIs (wedge rendering) and per-ROI statistics from the bitmap
follow in subsequent steps.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

viewer/image_viewer/JFJochDiffractionImage.cpp

@@ -313,6 +313,7 @@ void JFJochDiffractionImage::DrawTopPixels() {
 
 void JFJochDiffractionImage::addCustomOverlay() {
     DrawResolutionRings();
+    DrawROIs();
     DrawTopPixels();
 
     DrawBeamCenter();
@@ -326,6 +327,38 @@ void JFJochDiffractionImage::addCustomOverlay() {
     DrawResolutionText();
 }
 
+void JFJochDiffractionImage::DrawROIs() {
+    if (!image)
+        return;
+
+    const auto &rois = image->Dataset().experiment.ROI().GetROIDefinition();
+
+    // Distinct colours per ROI; loaded ROIs use solid lines (the interactively
+    // drawn scratch ROI keeps its dashed feature_color).
+    static const QColor palette[] = {Qt::cyan, Qt::yellow, QColor(0xff, 0x57, 0x22),
+                                     Qt::green, Qt::magenta, QColor(0x21, 0x96, 0xf3)};
+    const int palette_size = sizeof(palette) / sizeof(palette[0]);
+    int color_index = 0;
+    auto roi_pen = [&]() {
+        QPen pen(palette[color_index % palette_size], 2);
+        pen.setCosmetic(true);
+        color_index++;
+        return pen;
+    };
+
+    for (const auto &b : rois.boxes) {
+        auto *rect = scene()->addRect(b.GetXMin(), b.GetYMin(), b.GetWidth(), b.GetHeight(), roi_pen());
+        addOverlayItem(rect);
+    }
+
+    for (const auto &c : rois.circles) {
+        const float r = c.GetRadius_pxl();
+        auto *ell = scene()->addEllipse(c.GetX() - r, c.GetY() - r, 2 * r, 2 * r, roi_pen());
+        addOverlayItem(ell);
+    }
+    // Azimuthal ROIs are drawn as wedges in a later step.
+}
+
 
 void JFJochDiffractionImage::UpdateForeground() {
     if (!image || !auto_fg)

viewer/image_viewer/JFJochDiffractionImage.h

@@ -25,6 +25,7 @@ private:
     void addCustomOverlay() override;
     void LoadImageInternal();
     void DrawResolutionRings();
+    void DrawROIs();
     void DrawSpots();
     void DrawPredictions();
     void DrawBeamCenter();