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

include/plot/input/controller.hpp

@@ -59,9 +59,11 @@ enum class DragMode {
 /// therefore change its *physical* size with the display: a 5 px grab radius is
 /// half the hand movement on a 2x panel, so the same cursor becomes harder to
 /// catch there. Both tolerances -- the cursor grab radius and the box-zoom click
-/// threshold -- are therefore logical constants multiplied by `Plot::ui_scale()`,
-/// which is also why the controller reads the scale off the plot it already holds
-/// instead of keeping a copy a host would have to keep in step.
+/// threshold -- are therefore logical constants, converted through
+/// `Style::device()` at the scale of the plot the controller already holds: that is
+/// the single place the logical -> device rule lives, and the plot is the one that
+/// knows what its device pixel is worth, so the controller keeps no copy of the scale
+/// for a host to keep in step.
 ///
 /// The middle-drag axis zoom's per-pixel gain is deliberately *not* in that group:
 /// it is a data-navigation sensitivity rather than a size (the wheel and `+`/`-`