feat(plot): crosshair with an interpolated per-signal hover readout

include/plot/render/style.hpp

@@ -41,6 +41,13 @@ struct Style {
   Color axis_text = Color::hex(0xC8D0DC);
   Color cursor = Color::hex(0xFFB020);
   Color measure_fill = Color::hex(0xFFB020, 0.15f);
+  /// The crosshair's colour: its two lines, its markers' own colour excepted, and the
+  /// readout panel's border (spec §15). Deliberately **not** `cursor`: the cursor accent is
+  /// an orange that a caller's own cursors, a region's bounds and its fill all already use,
+  /// while the crosshair is a passive hover instrument that has to read as *chrome* over a
+  /// dense trace rather than as another handle to grab. A pale cyan does that against the
+  /// dark default theme, and it is a colour no default element of a frame shares.
+  Color crosshair = Color::hex(0xB8E8FF);
 
   // --- the scale ----------------------------------------------------------