Viewer: label pixels from the int32 image, and paint them instead of building items

Two changes to the per-pixel value labels, which appear above 30x zoom.

They were up to 5000 QGraphicsSimpleTextItems created and destroyed on every
overlay rebuild - so on every pan step while zoomed in. Paint them in
drawForeground() instead: no item churn, no scene invalidation, and the text is
laid out in viewport pixels so it is a constant readable size rather than a
scene-space font scaled by 0.2. Same approach as the magnifier's labels.

The value text becomes a virtual, PixelLabel(). The base still formats from
image_fp, which is what the genuinely float-valued views hold (azimuthal
profile, grid-scan 1/sigma^2, the calibration viewer's eight source types).
JFJochDiffractionImage overrides it to read the int32 image directly: counts are
exact integers, so routing them through float32 is a detour that also cannot
represent summed values above 2^24 exactly.

Verified at 38 wheel clicks over a module edge: identical values and
gap/contrast handling to the previous float path, now centred in each pixel.
Fit-view panel still pixel-identical to the pre-series baseline.

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

viewer/image_viewer/JFJochDiffractionImage.cpp

@@ -1011,6 +1011,20 @@ QString JFJochDiffractionImage::HoverResolutionLabel() const {
     return QString("d = %1 \u00C5").arg(QString::number(hover_resolution, 'f', 2));
 }
 
+QString JFJochDiffractionImage::PixelLabel(int x, int y) const {
+    if (!image)
+        return {};
+
+    const int32_t v = image->Image()[static_cast<size_t>(y) * W + x];
+    if (v == GAP_PXL_VALUE)
+        return QStringLiteral("Gap");
+    if (v == ERROR_PXL_VALUE)
+        return QStringLiteral("Err");
+    if (v == SATURATED_PXL_VALUE)
+        return QStringLiteral("Sat");
+    return QString::number(v);
+}
+
 void JFJochDiffractionImage::drawForeground(QPainter *painter, const QRectF &rect) {
     JFJochImage::drawForeground(painter, rect);