feat(render): logical style metrics with a UI scale, so chrome has a consistent physical size

include/plot/input/controller.hpp

@@ -50,6 +50,23 @@ enum class DragMode {
 ///
 /// A drag that changes no range is harmless: a box-zoom selection under 4 px on
 /// either side is a click, and leaves the ranges alone.
+///
+/// ## Tolerances are logical, the coordinates they are compared against are not
+///
+/// The plot geometry the controller hit-tests in is device pixels (see
+/// `PlotLayout`), and each adapter converts its pointer coordinates into the same
+/// space before forwarding them. A tolerance written as a device constant would
+/// 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.
+///
+/// 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 `+`/`-`
+/// are scale-free already, being counted in notches), and spec §12 reworks the
+/// zoom and pan gestures that own it.
 class Controller {
  public:
   explicit Controller(Plot& plot) noexcept : plot_(&plot) {}