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/input/controller.hpp

@@ -229,6 +229,18 @@ class Controller {
   DragMode drag_mode() const noexcept { return mode_; }
   /// The current box-zoom selection in pixels; meaningful while BoxZoom.
   Rect selection_rect() const noexcept { return selection_; }
+
+  /// The rectangle a host should *draw* for the open box zoom: the swept area, widened to the full
+  /// axes along whichever axis the modifier's mask will not zoom. Ctrl frames the x range only, so its
+  /// preview is the band that spans the plot -- which is what the release will zoom -- where the area
+  /// a purely horizontal drag sweeps is a sliver a few pixels tall. Shift (and any other combo) frames
+  /// both, and gets the swept box itself.
+  ///
+  /// Meaningful while `BoxZoom`, like `selection_rect()`; the zero rect otherwise.
+  ///
+  /// Defined in the implementation: widening the preview to the axes needs the plot's layout, and
+  /// `Plot` is only forward-declared here.
+  Rect selection_preview() const noexcept;
   /// The cursor under the pointer -- a vertical marker or a horizontal ruler, whichever
   /// line the pointer is nearer -- or `kInvalid` when the pointer is outside the plot area
   /// or no cursor is within a few pixels.