feat: an open box zoom draws the frame the release will apply

The controller has always tracked its selection; nothing drew it, so a Ctrl or
Shift drag framed a range with no feedback until the release.

The preview is the area the release will change, not what the pointer swept:
Ctrl frames the x range only, so a band across the plot is what it will zoom,
where the sliver a straight horizontal drag sweeps is only the gesture. The
plot carries the rect a host hands it -- it learns nothing about pointers --
and the renderer draws a low-alpha fill under a light border over everything
else inside the axes.

src/input/controller.cpp

@@ -751,6 +751,31 @@ void Controller::pan(const PointerEvent& e) {
   }
 }
 
+Rect Controller::selection_preview() const noexcept {
+  // Nothing to preview off a box zoom: the mask bits outlive the gesture -- `end_drag()` clears the
+  // selection, not them -- so widening without this guard would hand out a full-band rect for a drag
+  // that finished minutes ago.
+  if (mode_ != DragMode::BoxZoom) return Rect{};
+  // And nothing has been framed until the drag has swept every axis the release will apply: at the
+  // press instant the selection is an empty rect at the pointer, and the *widened* form of that is a
+  // band the width of the plot, which is a claim about a range that does not exist yet. Ctrl's other
+  // axis is deliberately not required: a drag with no vertical movement at all is a band across the
+  // plot, which is exactly what Ctrl is for.
+  if ((box_zoom_x_ && !(selection_.w > 0.0f)) || (box_zoom_y_ && !(selection_.h > 0.0f))) return Rect{};
+  Rect preview = selection_;
+  if (box_zoom_x_ && box_zoom_y_) return preview;
+  const Rect& axes = plot_->layout().axes;
+  if (!box_zoom_x_) {
+    preview.x = axes.x;
+    preview.w = axes.w;
+  }
+  if (!box_zoom_y_) {
+    preview.y = axes.y;
+    preview.h = axes.h;
+  }
+  return preview;
+}
+
 void Controller::box_zoom_update(const PointerEvent& e) {
   const f32 x = std::min(origin_x_, e.x);
   const f32 y = std::min(origin_y_, e.y);