feat(plot): horizontal rulers alongside the vertical cursors

include/plot/input/controller.hpp

@@ -81,7 +81,15 @@ using plot::RegionGrab;
 ///     nearest one winning. A marker is drawn on top of everything and is hit-tested
 ///     the same way: the shaded band between two measurement cursors is grabbed by
 ///     dragging either of the cursors that bound it, because its edges *are* those
-///     cursors -- no second path, no second tolerance.
+///     cursors -- no second path, no second tolerance. A *horizontal* ruler is the same
+///     target turned a quarter turn: the distance measured is the pointer's to the
+///     ruler's row rather than to a marker's column, and it is the same radius, so a
+///     point within reach of one of each resolves to the nearer and neither orientation
+///     widens the other's reach. The drag that follows writes only the coordinate the
+///     grabbed cursor's orientation owns -- `set_cursor_x()` for a marker,
+///     `set_cursor_y()` for a ruler -- and a dragged ruler is clamped to its own y
+///     axis' range, because a pointer past the top or bottom of `axes` has no data
+///     under it to follow (see `move_cursor()`).
 ///  2. **Regions next**, resolved nearest-first against the geometry `Plot::layout()`
 ///     measured: a bound within the grab radius beats the body (so the bound of a
 ///     wide region is grabbable even though the pointer is inside the fill), the
@@ -184,8 +192,9 @@ class Controller {
   DragMode drag_mode() const noexcept { return mode_; }
   /// The current box-zoom selection in pixels; meaningful while BoxZoom.
   Rect selection_rect() const noexcept { return selection_; }
-  /// The cursor under the pointer, or `kInvalid` when the pointer is outside the
-  /// plot area or no cursor is within a few pixels.
+  /// The cursor under the pointer -- a vertical marker or a horizontal ruler, whichever
+  /// line the pointer is nearer -- or `kInvalid` when the pointer is outside the plot area
+  /// or no cursor is within a few pixels.
   u64 hovered_cursor() const;
   /// The cursor a drag is moving, or `kInvalid` when no cursor drag is open. The two
   /// are the same answer the plot publishes for the renderer (see HotHandles), for a