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.

tests/test_render_smoke.cpp

@@ -5481,9 +5481,10 @@ plot::i32 densest_column(const std::vector<plot::u8>& frame, plot::u32 fb_w, plo
   return best;
 }
 
-/// The grid *majors* (0x2A3038 over the 0x101418 background), which no other element of these
-/// frames reaches: the minors are 0x1C2126 (too dark for the bound), the crosshair is near-white,
-/// a trace is saturated and the cursor is the orange accent.
+/// The grid *majors* (0x39414C over the 0x101418 background), which no other element of these
+/// frames reaches: the minors are that same colour at 50% alpha over a 0.75-covered dashed
+/// stroke, so their brightest pixel is about 15 levels off the background and stays under this
+/// bound; the crosshair is near-white, a trace is saturated and the cursor is the orange accent.
 bool grid_pixel(const plot::u8 px[4]) {
   const plot::u8 lo = std::min(px[0], std::min(px[1], px[2]));
   const plot::u8 hi = std::max(px[0], std::max(px[1], px[2]));