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.

include/plot/render/renderer.hpp

@@ -654,6 +654,11 @@ class Renderer {
   /// pointer is on: a readout the legend covered would be the feature failing at its one job.
   void draw_crosshair_readout(const Plot& plot, const PlotLayout& layout);
 
+  /// The host's box-zoom frame (`PlotLayout::selection`): a fill at `selection_fill` under a border at
+  /// `selection_edge`, or nothing at all when nothing was swept. Drawn last of the interior passes, so
+  /// the area being framed is legible over the traces, the regions and the readout alike.
+  void draw_selection_frame(const PlotLayout& layout);
+
   /// One marker shape (`Marker`) at a viewport-pixel centre: `Cross` two
   /// segments, `Circle` an 8-gon, `Square` four. Staged into the line batch in
   /// pixel space, at `size_px` half-extent, so a marker's size is device pixels