fix(render): convert Trace::width through the UI scale, and fold the controller's tolerance conversion into Style::device()

include/plot/plot/trace.hpp

@@ -11,6 +11,9 @@ namespace plot {
 struct TraceDesc {
   const data::Series* series = nullptr;
   Color color = Color::hex(0x4FA3FF);
+  /// The stroke's width in **logical** pixels, like every stroke in `Style` (see
+  /// that struct's unit rule): the renderer converts it once, so a trace has the
+  /// same weight against the axis lines whatever the display's DPI.
   f32 width = 1.5f;
   u32 axis_x = 0, axis_y = 0;
   /// What the trace is called, for the legend. Empty means "do not list it", which
@@ -28,6 +31,8 @@ struct TraceDesc {
 struct Trace {
   const data::Series* series = nullptr;
   Color color = Color::hex(0x4FA3FF);
+  /// Logical pixels, as in `TraceDesc::width`: converted to device pixels by the
+  /// renderer's one `Style::device()` call per stroke.
   f32 width = 1.5f;
   u32 axis_x = 0, axis_y = 0;
   /// The legend's entry for this trace, empty when it has none. A name does not