style(grid): the minor grid is thinner, dashed and blended into the background

Subdivisions used to be drawn exactly like the labelled lines -- same width, same
opacity -- so a plot dense enough to need them read as two competing sets of ticks.
The minor class is now three things the style states explicitly:

- grid_minor_width 1.0 -> 0.75, a sub-pixel logical width. At 1x that is expressed as
  reduced coverage through the anti-aliased ramp; on a HiDPI display the line is
  literally narrower.
- grid_minor_dash_on/_off (new, 3/3): dashed by default, off == 0 draws it solid, the
  same convention as the crosshair's and a trace's dash. The dash is a fragment-shader
  discard, so no geometry is added -- the vertices are the line either way.
- grid_minor carries alpha (0x1C2126 at 50%), and the line shader applies u_color.a,
  so it blends into the background instead of sitting on it.

Together that is about a fifth of the ink a major line leaves along its length, which
is what gives the eye one grid to read instead of two. The major class is untouched:
it stays the structure.

Tests: a new pixel test (the_minor_grid_is_thinner_dashed_and_dimmer_than_the_major)
asserts a major column is inked top-to-bottom while a minor one alternates, and that
the minor's ink sits closer to the background than the major's. Three existing tests
counted the minor class as "most of eight rows" of its exact colour; two now take
their floor from the style (minor_line_ink: width x alpha x duty), and the width test
states the dash and alpha away because widths are its subject. 361 cases, 0 failed.

README.md

@@ -30,7 +30,9 @@ run touched a GPU, so read it as an upper bound and a record of the software pat
 ## Features
 
 - grid with independent major/minor line classes (`Style::grid_major_visible`,
-  `grid_minor_visible`);
+  `grid_minor_visible`), each with its own colour, width and (for the minors) dash — the
+  subdivisions default to thinner, dashed and alpha-blended, so they read as texture under the
+  labelled grid rather than as a second set of ticks;
 - x and y axes, with a vertical (bottom-to-top) y-axis title;
 - cursors in both orientations — vertical markers and horizontal rulers;
 - draggable regions and measurement bands with exact interval statistics;