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.

src/plot/plot.cpp

@@ -311,6 +311,12 @@ void Plot::set_cursor_color(u32 id, Color color) {
   cursors_[id].color = color;
 }
 
+void Plot::set_cursor_stroke(u32 id, f32 width, Vec2 dash) {
+  if (id >= cursors_.size()) return;
+  cursors_[id].width = width;
+  cursors_[id].dash = dash;
+}
+
 void Plot::set_cursor_selectable(u32 id, bool selectable) {
   if (id >= cursors_.size()) return;
   cursors_[id].selectable = selectable;