feat(plot): crosshair with an interpolated per-signal hover readout

include/plot/input/controller.hpp

@@ -124,6 +124,18 @@ using plot::RegionGrab;
 /// the same answer for a host that wants it directly -- a resize cursor, a tooltip,
 /// a status line.
 ///
+/// ## The crosshair (spec §15)
+///
+/// A pointer over the data area places the plot's crosshair at the data coordinates under
+/// it (`Plot::set_crosshair()`), and a pointer that leaves `axes` clears it
+/// (`Plot::clear_crosshair()`). That is a *hover*, not a gesture: no button is involved, and
+/// a press, a wheel notch or a key stroke that moves the data under the pointer re-anchors
+/// the crosshair to the same pointer position rather than clearing it -- it follows the
+/// pointer, so anything the pointer did not move leaves it where the pointer is. The
+/// per-signal values a readout shows are the plot's (`PlotLayout::crosshair`), not this
+/// layer's: the controller says where the pointer is and nothing about what a crosshair
+/// means.
+///
 /// ## Zoom and pan on a live x axis (spec §12)
 ///
 /// An x axis armed with `XFollow::Live` keeps following through every gesture
@@ -262,6 +274,16 @@ class Controller {
   /// leave alone, the bounds and the grabbed data x a body move needs to preserve
   /// the width -- so a changed pointer motion never accumulates error.
   bool begin_handle_drag(const PointerEvent& e);
+  /// Places the plot's crosshair under the pointer, or clears it when the pointer is not
+  /// over the data area (spec §15). Called after every event that can move the pointer *or*
+  /// the data under it -- a pointer event, a wheel notch, a key stroke, a viewport change --
+  /// so the crosshair is where the pointer is and reads the data the frame will draw, and
+  /// never at a coordinate a zoom has already moved out from under it.
+  ///
+  /// A press does not clear it: the crosshair follows the pointer, and a pointer that is
+  /// still over the axes is still pointing at whatever gesture just started. The pointer is
+  /// the only thing that clears it, by leaving `axes`.
+  void update_crosshair(f32 px, f32 py);
   /// Publishes what the pointer is on to the plot, for the renderer to highlight
   /// (see HotHandles). Called after every event that can move the geometry under the
   /// pointer -- a pointer event, a wheel notch, a key stroke -- so hover and grab are