feat(render): independent major and minor grid classes tied to the tick set

include/plot/render/renderer.hpp

@@ -148,8 +148,9 @@ class Renderer {
   /// Every metric here is a logical unit (`Style`'s unit rule): the host writes
   /// `ui_scale` from the display it is on and this converts each metric with
   /// `device()`. `line_width_px` is the width of the strokes framing the data (axis
-  /// lines, cursor markers) -- grid lines stay hairlines and every trace carries its
-  /// own width.
+  /// lines, cursor markers); the grid carries a width *per class*
+  /// (`grid_major_width` / `grid_minor_width`) and every trace its own, and a grid
+  /// class the style switches off stages no geometry at all.
   render::Style& style() noexcept { return style_; }
   const render::Style& style() const noexcept { return style_; }