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.

docs/interaction.md

@@ -192,6 +192,16 @@ without knowing what a pointer is. `hovered_cursor()`, `grabbed_cursor()`, `hove
 `grabbed_region()` and `region_grab(u32)` expose the same answers to a host (a resize cursor, a
 tooltip, a status line).
 
+The frame an open box zoom is drawing is published the same way and for the same reason
+(`Plot::set_selection_rect()`), and the plot still learns nothing about pointers: the host reads the
+rect off the controller -- `Controller::selection_preview()` -- on every frame of the drag and takes it
+off the plot when the gesture ends. The preview is deliberately not what the pointer swept. Shift
+frames both axes and its preview is the swept box; Ctrl frames x only, so its preview is widened to the
+full height of the axes, and Alt's to the full width. The swept sliver is the *gesture*; the widened
+rect is what the *release* will apply, which is the one worth showing while it is still only a
+gesture. Nothing is previewed until the drag has swept every axis the release will act on: an empty
+rect widened is a band the width of the plot, which would claim a range that does not exist yet.
+
 ## Live follow
 
 An x axis has a follow state, `XFollow::Off` (the default) or `XFollow::Live`