feat: a repaired invalid span is shaded faintly, not solidly

include/plot/render/renderer.hpp

@@ -513,10 +513,11 @@ class Renderer {
   void draw_plot(Plot& plot, const PlotLayout& layout);
 
   void draw_grid(const Plot& plot);
-  /// `Plot::invalid_spans()`, as `PlotLayout::invalid_spans` measured them: one fill per span in
-  /// `style_.invalid_fill`. Over the grid and under everything else inside the axes, so a hole
-  /// reads as a property of the plot's background -- the grid shows through it and a trace that
-  /// does have data in those columns is drawn on top, which is what distinguishes a reserved
+  /// `Plot::invalid_spans()`, as `PlotLayout::invalid_spans` measured them: one fill per span,
+  /// in `style_.invalid_fill` or — for a span the host has marked repaired —
+  /// `style_.invalid_repaired_fill`. Over the grid and under everything else inside the axes, so
+  /// a hole reads as a property of the plot's background -- the grid shows through it and a trace
+  /// that does have data in those columns is drawn on top, which is what distinguishes a reserved
   /// hole from a trace that has gone quiet.
   void draw_invalid_spans(const PlotLayout& layout);
   void draw_axis_lines(const Plot& plot);

include/plot/render/style.hpp

@@ -62,6 +62,13 @@ struct Style {
   /// low enough that the grid under it stays legible, which is what makes the band read as a
   /// region of the plot rather than as a panel laid over it.
   Color invalid_fill = Color::hex(0x8A94A6, 0.18f);
+  /// The same shading for a span whose samples have since arrived and been written in, at well
+  /// under half the alpha. The gap is still reported -- the window was drawn without that data --
+  /// but a reader can tell a hole that is still empty from one that has been filled. Fainter by
+  /// alpha alone and not by hue, so the two read as two depths of one mark rather than as two
+  /// different kinds of thing; and still above the threshold where the difference from *no* band
+  /// is visible, which is the floor this alpha is chosen against.
+  Color invalid_repaired_fill = Color::hex(0x8A94A6, 0.08f);
   /// The crosshair's colour: its two lines and the readout panel's border (spec §15). Markers are
   /// deliberately *not* this colour -- they carry each trace's own, because a marker says *which*
   /// trace and the line says *where*.