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

include/plot/plot/plot.hpp

@@ -591,6 +591,17 @@ class Plot {
   /// **not** part of `set_crosshair()`, so placing a hover cannot silently change it.
   void set_crosshair_readout(bool on) noexcept { crosshair_.show_readout = on; }
 
+  /// Overrides this plot's crosshair stroke and dash, in logical pixels, without touching the
+  /// global style every other plot draws with (spec §17): each field left negative in `s`
+  /// defers to the matching `Style::crosshair_*` metric, so `CrosshairStyle{}` (the default)
+  /// is exactly the global style. Set it once, like a colour -- it is not part of
+  /// `set_crosshair()`, so placing a hover cannot silently change how the stroke looks. A
+  /// `dash_off` that resolves to `0` draws a solid line.
+  void set_crosshair_style(const CrosshairStyle& s) noexcept { crosshair_.style = s; }
+  /// The crosshair's style override, as set -- read back rather than the *resolved* stroke and
+  /// dash, which live on `CrosshairLayout` because they depend on `ui_scale`.
+  const CrosshairStyle& crosshair_style() const noexcept { return crosshair_.style; }
+
   /// The crosshair's state: where it is, which lines it draws and whether the readout is
   /// on. Read-only -- `set_crosshair()` and `clear_crosshair()` are how it moves -- so the
   /// coordinates it holds and the geometry `layout()` places from them cannot disagree.