style(grid): brighter major grid

grid_major 0x2A3038 -> 0x39414C, still well under axis_line's 0x8A94A6, so the three-way
hierarchy the eye reads stays minor < major < axis. Already fully opaque, so "more
prominent" meant a brighter colour and nothing else.

The tests that classify grid pixels take their colours from the style, so they followed
the change; the one hardcoded mention (grid_pixel()'s doc comment) is updated, and the
minor's relationship to the major is now stated as an order of magnitude rather than the
fifth the old colours happened to give. 361 cases, 0 failed.

include/plot/render/style.hpp

@@ -38,13 +38,18 @@ struct Style {
   // --- colours: neither logical nor device, so nothing to scale ------------
 
   Color background = Color::hex(0x101418);
-  Color grid_major = Color::hex(0x2A3038);
+  /// The labelled grid. Brighter than the subdivisions and still well under `axis_line`
+  /// (0x8A94A6): the grid is structure the eye can read, the axis is the frame, and the
+  /// hierarchy between the three is what keeps a dense plot legible. It is already fully
+  /// opaque, so "more prominent" means a brighter colour and nothing else.
+  Color grid_major = Color::hex(0x39414C);
   /// The minor grid's colour, **with an alpha** so it blends into the background rather than
   /// sitting on it: a subdivision is a reading aid, and at full opacity a dense minor grid
   /// reads as a second set of ticks competing with the labelled one. The line shader applies
   /// `u_color.a` (see `src/render/shaders.cpp`), so this one blends and the major -- opaque at
-  /// 0x2A3038 -- stays the structure the eye picks out. Together with `grid_minor_width` and
-  /// the dash below, the defaults put a minor line at roughly a fifth of a major line's ink.
+  /// 0x39414C -- stays the structure the eye picks out. Together with `grid_minor_width` and
+  /// the dash below, the defaults put a minor line at an order of magnitude less ink than a
+  /// major one.
   Color grid_minor = Color::hex(0x1C2126, 0.5f);
   Color axis_line = Color::hex(0x8A94A6);
   Color axis_text = Color::hex(0xC8D0DC);