refactor(render): share the readout panel and swatch drawing with the legend

include/plot/render/renderer.hpp

@@ -487,6 +487,18 @@ class Renderer {
   /// them again, so the panel a test asserts and the panel this draws are the same
   /// numbers.
   void draw_legend(const Plot& plot, const PlotLayout& layout);
+  /// The backplate the legend and the crosshair's readout share: the frame's background at
+  /// `kLegendPanelAlpha` over `panel`, with a one-device-pixel border in `border` around the
+  /// outermost pixels the fill covers (the crisp-hairline rule -- a border drawn on the measured
+  /// rect's fractional edges is lit half-heartedly, and a frame bright on three sides and dim on the
+  /// fourth reads as a mistake). One helper, two callers: the border's colour is the only thing a
+  /// legend panel and a readout panel do not share.
+  void draw_readout_panel(const Rect& panel, Color border);
+  /// One trace's colour swatch in `box`, opaque whatever the trace's own alpha is: a swatch drawn at
+  /// a trace's 15% would be the least legible thing in the frame, and the swatch is the one element
+  /// that says which colour an entry or a readout row *is*. The legend's entries and the crosshair's
+  /// rows both read it from here.
+  void draw_swatch(const Plot& plot, u32 trace, const Rect& box);
   /// The crosshair's **lines and markers**: the vertical and horizontal strokes and a small
   /// cross at each trace's intersection.
   ///