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

include/plot/render/style.hpp

@@ -55,6 +55,17 @@ struct Style {
   f32 axis_font_px = 12.0f;        ///< logical em the axis text is drawn at.
   f32 grid_label_pad = 4.0f;       ///< logical gap between a tick label and the axes,
                                    ///< and the inset inside a title strip.
+  /// The two grid classes, each with its own width and its own switch. The grid's
+  /// *positions* come from the tick set alone -- `Grid::build()` tags every segment
+  /// with the class of the tick it came from -- so these four decide only how a
+  /// class is drawn. A class switched off stages no geometry at all: the renderer
+  /// skips it before the first segment is staged, so a dense plot's minor grid
+  /// costs nothing rather than being drawn transparent (spec §11). The colours
+  /// above are already per class.
+  f32 grid_major_width = 1.0f;     ///< logical width of a major (labelled-tick) line.
+  f32 grid_minor_width = 1.0f;     ///< logical width of a minor (subdivision) line.
+  bool grid_major_visible = true;  ///< stage the major grid at all.
+  bool grid_minor_visible = true;  ///< stage the minor grid at all.
   f32 line_width_px = 1.5f;        ///< logical width of the strokes framing the data
                                    ///< (axis lines, cursor markers).
   f32 tick_min_px = 60.0f;         ///< logical minimum spacing between major ticks, so