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

include/plot/render/renderer.hpp

@@ -487,6 +487,18 @@ class Renderer {
   /// them again, so the panel a test asserts and the panel this draws are the same
   /// numbers.
   void draw_legend(const Plot& plot, const PlotLayout& layout);
+  /// The crosshair: the two lines, a marker per trace, and the readout panel with a swatch and
+  /// a value per listed row. Every rect -- the two stroke boxes, the panel, the markers'
+  /// positions and the rows' boxes -- comes from `PlotLayout::crosshair`, which placed it
+  /// inside `axes`; this stages exactly that geometry, so the crosshair a test measures and the
+  /// crosshair a frame paints are one set of numbers. Clipped to `axes` with the rest of the
+  /// interior passes, which is what keeps a marker for an off-scale sample inside the rect.
+  ///
+  /// The panel is a backplate like the legend's, so it is drawn after the lines and the
+  /// markers and before any of the frame's text. The rows' values were formatted by the plot
+  /// (in the trace's own y axis' format) and are staged here as they stand -- this layer
+  /// formats nothing.
+  void draw_crosshair(const Plot& plot, const PlotLayout& layout);
 
   /// The built-in backend the renderer owns, and the face the frames draw with:
   /// `set_text_backend()` writes `font_`, which starts here and comes back here

include/plot/render/style.hpp

@@ -41,6 +41,13 @@ struct Style {
   Color axis_text = Color::hex(0xC8D0DC);
   Color cursor = Color::hex(0xFFB020);
   Color measure_fill = Color::hex(0xFFB020, 0.15f);
+  /// The crosshair's colour: its two lines, its markers' own colour excepted, and the
+  /// readout panel's border (spec §15). Deliberately **not** `cursor`: the cursor accent is
+  /// an orange that a caller's own cursors, a region's bounds and its fill all already use,
+  /// while the crosshair is a passive hover instrument that has to read as *chrome* over a
+  /// dense trace rather than as another handle to grab. A pale cyan does that against the
+  /// dark default theme, and it is a colour no default element of a frame shares.
+  Color crosshair = Color::hex(0xB8E8FF);
 
   // --- the scale ----------------------------------------------------------