Viewer: rate-limit hover feedback to ~15 Hz

The status bar, the resolution readout and the magnifier were all regenerated
on every single mouse motion event. Each regeneration repaints, and on a remote
X session a repaint uploads the whole window regardless of how little changed,
so the pointer merely crossing the image saturates the link: measured with a
counting relay in front of the X server, 50 motions over the image cost 273 MB,
and a build with the hover work removed cost 18 KB.

Rate-limit it. Two details matter:

- The limit is applied inline, not from a timer. Running the update inside the
  mouse event keeps its damage in the same repaint as anything else that event
  triggers (a pan). A first attempt deferred the work to a timer instead, which
  split one repaint into two and made panning measurably worse.
- The catch-up that reports the final position is debounced, not queued per
  skipped motion, so it fires once after the pointer stops rather than
  repeatedly mid-gesture.

mouseHover() now takes the scene position and modifiers instead of the event,
which also removes the identical mapToScene() from all four implementations.

Hover traffic over 3 repeats: 173 MB mean -> 140 MB, and the run-to-run spread
drops from +-14% to +-2%. The harness tops out near 30 motions/s, barely above
the 15 Hz limit; a real mouse reports far faster, where the cap does more.

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

viewer/image_viewer/JFJochDiffractionImage.cpp

@@ -79,8 +79,7 @@ void JFJochDiffractionImage::azimuthalHandles(const ROIAzimuthal &az, const Diff
     phimax = pt(d_mid, phi1);
 }
 
-void JFJochDiffractionImage::mouseHover(QMouseEvent *event) {
-    auto coord = mapToScene(event->pos());
+void JFJochDiffractionImage::mouseHover(const QPointF &coord, Qt::KeyboardModifiers) {
 
     if (image && (coord.x() >= 0)
         && (coord.x() < image->Dataset().experiment.GetXPixelsNum())