feat(plot): the figure crosshair marks the hovered row, and the stroke is thin, dashed and configurable

include/plot/render/style.hpp

@@ -54,6 +54,21 @@ struct Style {
   /// already distinct, so no dashes.
   Color crosshair = Color::hex(0xE6EBF2);
 
+  /// The crosshair's stroke and dash, in logical pixels (spec §17). The crosshair is a
+  /// *passive* hover instrument drawn over the data, so its default **recedes**: a
+  /// thinner stroke than a trace's (`Trace::width`'s 1.5) so it reads as chrome rather
+  /// than as data, and a dash so it is distinguishable from the grid and the traces at
+  /// a glance.
+  ///
+  /// `crosshair_dash_on` is the ink length and `crosshair_dash_off` the gap; **`off == 0`
+  /// means solid**, which is the value every *other* line in the library is drawn at, so
+  /// the one pipeline serves them unchanged (the dash is a fragment-shader discard on
+  /// `v_along`, so no geometry is added). A `Crosshair` may override all three for one
+  /// plot without touching this global style.
+  f32 crosshair_width = 0.75f;
+  f32 crosshair_dash_on = 4.0f;
+  f32 crosshair_dash_off = 4.0f;
+
   // --- the scale ----------------------------------------------------------
 
   /// Device pixels per logical unit. 1.0 is the 1x display every default here was