feat(plot): the figure crosshair marks the hovered row, and the stroke is thin, dashed and configurable

include/plot/render/renderer.hpp

@@ -365,7 +365,12 @@ class Renderer {
   /// Uploads the staged line batch, maps it with (`scale`, `offset`), rasterises
   /// it in one colour and empties the batch. A no-op for an empty batch, so the
   /// caller can flush unconditionally.
-  void flush_lines(const Mapping& map, Color color, f32 width) noexcept;
+  ///
+  /// `dash` is the pattern in pixels for the fragment shader's discard: `x` the ink
+  /// length, `y` the gap, and a non-positive `y` -- the default every caller but the
+  /// crosshair passes -- means **solid**. It is set on every flush, so one draw's dash
+  /// can never leak into the next.
+  void flush_lines(const Mapping& map, Color color, f32 width, Vec2 dash = Vec2{}) noexcept;
   /// Draws every staged glyph in `style().axis_text` and empties the batch. Text
   /// is texture-bound, so this runs once per frame, after all line geometry.
   void flush_text() noexcept;