fix(plot,render,glfw): a hue-neutral crosshair, an em-dash placeholder, one layout per figure event, and a usable readout switch

include/plot/render/style.hpp

@@ -41,13 +41,18 @@ 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 crosshair's colour: its two lines and the readout panel's border (spec §15). Markers are
+  /// deliberately *not* this colour -- they carry each trace's own, because a marker says *which*
+  /// trace and the line says *where*.
+  ///
+  /// **Hue-neutral by design**, and that is the whole choice: a saturated colour here would be
+  /// confusable with a trace, and the library's own default trace colour is a saturated cyan
+  /// (`Trace`'s default, 0x40C0FF) that a hue-based reading cannot tell from a cyan crosshair --
+  /// only lightness would separate them, which no reader can rely on. A near-white can never be a
+  /// saturated trace of *any* hue, and it is still plainly not `cursor`'s orange, which is what
+  /// the spec asks for. The 1 px stroke is kept: form (a full-height thin line over a polyline) is
+  /// already distinct, so no dashes.
+  Color crosshair = Color::hex(0xE6EBF2);
 
   // --- the scale ----------------------------------------------------------