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

adapters/gtk4/gtk_plot_area.cpp

@@ -303,6 +303,12 @@ void PlotArea::render() noexcept {
   last_render_s_ = started;
   if (frame_) frame_(*plot_, dt);
 
+  // The mapping's factor is the display's scale, so the renderer draws its logical
+  // style metrics at the same ratio the viewport and every forwarded coordinate are
+  // in: a 12 px label is the same physical size on a 1x and a 2x widget. It is the
+  // `::resize` ratio, not `gtk_widget_get_scale_factor()` -- the latter is the
+  // ceiling under fractional scaling and would over-scale the chrome by up to 60%.
+  renderer_->style().ui_scale = mapping_.factor;
   renderer_->begin_frame(static_cast<u32>(viewport.w), static_cast<u32>(viewport.h), viewport);
   renderer_->draw(*plot_);
   renderer_->end_frame();

adapters/gtk4/gtk_plot_area.hpp

@@ -110,11 +110,14 @@ using FrameFn = std::function<void(Plot&, f64 dt)>;
 /// a delta is then a logical difference scaled by the factor in force now, never
 /// the difference of two device positions (see `DragAnchor`).
 ///
-/// Note that this also means the renderer's *style metrics* stay in device
-/// pixels: at scale 2 the label font is 24 device px = 12 logical px, so labels
-/// are smaller relative to the widget than at 1x. That is the same convention
-/// the GLFW and Qt adapters use, and a host that wants a constant logical text
-/// size scales `Renderer::style()` metrics by the factor itself.
+/// The same factor states the display to the *renderer*: it is written to
+/// `Renderer::style().ui_scale` every frame, which is what makes the chrome the
+/// same physical size at any scale. The style's metrics are logical (see the unit
+/// rule on `render::Style` -- spec §10 reverses the older convention that had them
+/// in device pixels and accepted smaller labels on a high-DPI host), so at scale 2
+/// the label font is 12 logical px drawn at 24 device px: the same size on screen as
+/// at 1x, not a smaller one. This is the same convention the GLFW, Qt and ImGui
+/// adapters use, and none of them has to scale a style metric by hand.
 ///
 /// ## Input
 ///