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

include/plot/plot/grid.hpp

@@ -12,16 +12,26 @@ namespace plot {
 /// baking screen coordinates in here would break that uniform-only contract.
 struct GridLine {
   f64 x0 = 0.0, y0 = 0.0, x1 = 0.0, y1 = 0.0;
+  /// The class of the tick this segment came from: a major (labelled) tick, or one
+  /// of the minor subdivisions between them. The renderer picks the class' colour
+  /// and width from it, so the tick set is the one source for grid positions and
+  /// `Style` the one place that decides how a class looks and whether it is drawn.
   bool major = true;
 };
 
 /// Tick-aligned grid geometry: one segment per tick, spanning the perpendicular
 /// axis' full range.
 class Grid {
+///
+/// Every tick within the perpendicular range contributes a segment, whatever its
+/// class, tagged by it. Whether a class is *drawn* is style rather than geometry:
+/// the renderer checks `Style::grid_major_visible` / `grid_minor_visible` per class
+/// before it stages a segment, which is the only switch for that behaviour (spec
+/// §11). A `Grid` therefore carries both classes' geometry whatever the style says,
+/// and a caller that draws it itself applies the same two switches.
  public:
   bool vertical = true;    ///< emit lines at x ticks
   bool horizontal = true;  ///< emit lines at y ticks
-  bool show_minor = true;  ///< keep minor-tick lines
 
   /// Rebuilds the segments from each axis' ticks and range. Ticks outside the
   /// perpendicular range (or non-finite) are skipped, so a stale tick list can