fix(plot,glfw): layer the crosshair under the handles, sync it across a figure, and harden the lookup

adapters/glfw/glfw_host.hpp

@@ -107,6 +107,12 @@ class Host {
   /// `Renderer::draw(Figure&)`. The frame callback is handed the figure's **row 0**
   /// as its `Plot&`, so `plot()` and the frame argument agree.
   ///
+  /// The figure's **crosshair** follows the pointer as one crosshair and not one per row: the run
+  /// loop reads the hover out of the row under the pointer and applies it to the whole figure
+  /// (`sync_figure_crosshair()`), so every row's readout lists its own signals at the one x the
+  /// pointer is over. That is what "synced if multiple plots available" means, and the host is the
+  /// layer that can do it -- it is the one that knows which row the pointer is in.
+  ///
   /// The figure is borrowed and must outlive the run. Passing null restores the
   /// single-plot loop, which is what every other caller uses; the host's own
   /// `plot_` is then untouched by any of this.
@@ -168,6 +174,28 @@ 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;
+
+  /// Spreads the hover over a whole `Figure` (spec §15).
+  ///
+  /// A figure's rows share one x, so the crosshair one row's controller placed is read back here and
+  /// applied to every row: the controller turns a pointer position into a data coordinate -- which
+  /// is its job, and the same one it does for a single plot -- and the figure spreads that one
+  /// coordinate over its rows. Reading it back rather than deriving a second coordinate here is what
+  /// keeps the hover the row shows and the hover the figure shows the same number.
+  ///
+  /// The row it reads is the one under the host's pointer, or -- with no pointer of the host's own,
+  /// which is how a scripted run injects its events -- the row whose crosshair is not the value this
+  /// host spread last (`spread_x_`/`spread_active_`), i.e. the row something outside changed. A row
+  /// 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;
+
   Plot plot_;
   input::Controller controller_;  ///< forwards its viewport to `plot_`
   std::optional<Renderer> renderer_;
@@ -183,6 +211,13 @@ class Host {
   /// no position of its own, and the row the pointer is over is the one the user is
   /// looking at.
   u32 last_row_ = 0u;
+  /// The crosshair this host last spread over the figure (see
+  /// `sync_figure_crosshair()`): what a row is compared against to find the one a
+  /// scripted run touched, since the figure writes one value to every row.
+  /// `spread_active_` false means "spread nothing" -- the state before the first
+  /// sync, and the one a clear leaves.
+  f64 spread_x_ = 0.0;
+  bool spread_active_ = false;
 
   u32 fb_width_ = 0;   ///< framebuffer size of the last refresh_sizes()
   u32 fb_height_ = 0;