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

include/plot/render/style.hpp

@@ -55,6 +55,14 @@ struct Style {
   Color axis_text = Color::hex(0xC8D0DC);
   Color cursor = Color::hex(0xFFB020);
   Color measure_fill = Color::hex(0xFFB020, 0.15f);
+  /// The box-zoom selection's frame, drawn while a modified left drag is choosing a range: a light
+  /// neutral near full alpha for the border and the same neutral at a low one for the fill. Neutral
+  /// and not a trace colour, on `invalid_fill`'s argument -- the frame says "this is the area", and a
+  /// tint a trace could have would read as data. The fill's alpha is chosen against the same floor:
+  /// low enough that what is under the frame stays readable, because the point is to show what is
+  /// being selected, not to hide it.
+  Color selection_fill = Color::hex(0xC8D0DC, 0.12f);
+  Color selection_edge = Color::hex(0xC8D0DC, 0.85f);
   /// The shading over a span the host has declared invalid (`Plot::set_invalid_spans()`): the
   /// axis furniture's own neutral grey at a low alpha, on the same model as `measure_fill`
   /// above. Neutral and not a trace colour on purpose -- a band is the *absence* of data, and