fix(plot,render,glfw): a hue-neutral crosshair, an em-dash placeholder, one layout per figure event, and a usable readout switch

adapters/glfw/glfw_host.hpp

@@ -174,10 +174,11 @@ class Host {
   void on_scroll(f64 dx, f64 dy) noexcept;
   void on_key(int key, int action, int mods) noexcept;
 
-  /// The index of the figure row a framebuffer pixel belongs to, by the rule
-  /// `controller_at()` routes by: the row the pixel is in, or the nearest row when it is in none
-  /// (a row gap, or a margin outside the figure). 0 while no figure is set.
-  u32 row_at(f32 x, f32 y) const noexcept;
+  /// The index of the row of `rows` a framebuffer y belongs to: the row it is in, or the nearest
+  /// row when it is in none (a row gap, a margin outside the figure). One rule, shared by the event
+  /// routing and the crosshair sync -- and taking the rects rather than reading them, so a caller
+  /// that has just laid the figure out does not make it lay out again for this.
+  static u32 row_in(const std::vector<Rect>& rows, f32 y) noexcept;
 
   /// Spreads the hover over a whole `Figure` (spec §15).
   ///
@@ -193,8 +194,9 @@ class Host {
   /// whose controller has no crosshair clears the figure's.
   ///
   /// Called once per frame from the run loop, so a scripted run that bypasses the GLFW callbacks
-  /// gets the same synced figure a real pointer does.
-  void sync_figure_crosshair() noexcept;
+  /// gets the same synced figure a real pointer does. `rows` is the figure's row rects, which the
+  /// run loop has just read -- the one layout per frame this shares with the row viewports.
+  void sync_figure_crosshair(const std::vector<Rect>& rows) noexcept;
 
   Plot plot_;
   input::Controller controller_;  ///< forwards its viewport to `plot_`