feat(cursor): a cursor's stroke is its own -- width and dash

Cursors took `Style::line_width_px` and were always solid, so a host had no way to place a
*readout mark* on the data (a measured level, a peak, a pulse's edge) without it reading as
another instrument framing the plot. `set_cursor_stroke(id, width, dash)` gives it its own, the
same shape as `set_cursor_color()`: per cursor, because one plot holds both kinds.

`width <= 0` means the style's default for that cursor (`handle_width_px` while hovered or
grabbed, `line_width_px` otherwise), and a hover can only make the stroke *thicker* -- the cue
that the pointer has hold of something must not be the thing that hides it. `dash.off <= 0` is
solid, the convention a trace's, the crosshair's and the grid's dash already share.

Test: a_cursor_stroke_can_be_thinner_and_dashed -- a default cursor and a thin dashed one in the
same frame. The dashed column alternates ink and gap (153/151 at the 4/4 the test asks for)
while the solid one has no gaps; the ink laid down over each stroke's whole run of columns is
less for the thin one. That last assertion is deliberately *not* a single-column peak: a cursor
is a data position and is not snapped, so the solid stroke's ink split across two columns and
read *lighter* than the thin one -- which is how this test first failed.

include/plot/plot/plot.hpp

@@ -484,6 +484,10 @@ class Plot {
   /// be told apart from its neighbours, and its colour is how it says which readout it
   /// belongs to. An id that names no cursor is ignored, like every other setter here.
   void set_cursor_color(u32 id, Color color);
+
+  /// A cursor's stroke: its logical width (`<= 0` for the style's default) and its dash
+  /// (`dash.off <= 0` for solid). The instrument's own, like its colour -- see `Cursor::width`.
+  void set_cursor_stroke(u32 id, f32 width, Vec2 dash);
   /// Makes a cursor unselectable (see `Cursor::selectable`): the pointer cannot hover or
   /// grab it, which is what a host wants for a marker it draws as a *readout*. An id that
   /// names no cursor is ignored, like every other setter here.