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.

docs/styling.md

@@ -36,7 +36,7 @@ Colours (neither logical nor device — nothing to scale):
 | field | default | meaning |
 |---|---|---|
 | `background` | `0x101418` | frame clear colour and legend/readout panel base |
-| `grid_major` | `0x2A3038` | major (labelled-tick) grid lines |
+| `grid_major` | `0x39414C` | major (labelled-tick) grid lines |
 | `grid_minor` | `0x1C2126` at 50% alpha | minor (subdivision) grid lines |
 | `axis_line` | `0x8A94A6` | axis lines |
 | `axis_text` | `0xC8D0DC` | tick labels and axis titles |
@@ -99,7 +99,7 @@ grid's three defaults are what makes that read: it is thinner than the major hai
 display as a narrower line), its colour carries an **alpha** so it blends into the
 background instead of sitting on it, and it is **dashed** (`grid_minor_dash_on`/`_off`) so an
 unlabelled line at an unlabelled position is never mistaken for a labelled one. Together
-those put a minor line at roughly a fifth of a major line's ink — at full opacity and the
+those put a minor line at an order of magnitude less ink than a major one — at full opacity and the
 same width a dense minor grid reads as a second set of ticks. The dash is a fragment-shader
 discard, not extra geometry: the vertices are the line either way. Set
 `grid_minor_dash_off = 0` for the old solid subdivision.