feat(input): a cursor can be unselectable, for a host's readout marks

A host that draws the result of a computation as a cursor -- a ruler at a level
it measured, a marker at a time it found -- must not let the user drag it: the
value belongs to the measurement, not to the pointer. Parking it outside the
axis and re-placing it every frame does not achieve that, because a drag is held
for the gesture's life and the marker follows the pointer between frames, so the
readout *looks* draggable however often it is put back.

`Cursor::selectable` defaults true; the controller's hit test skips an
unselectable cursor, so it is neither hovered nor grabbed and its hit radius
goes to whatever is behind it. Set through `Plot::set_cursor_selectable()`.

Tested by `an_unselectable_cursor_is_neither_hovered_nor_grabbed` in
tests/test_controller.cpp.

include/plot/plot/plot.hpp

@@ -484,6 +484,10 @@ class Plot {
   /// be told apart from its neighbours, and its colour is how it says which readout it
   /// belongs to. An id that names no cursor is ignored, like every other setter here.
   void set_cursor_color(u32 id, Color color);
+  /// Makes a cursor unselectable (see `Cursor::selectable`): the pointer cannot hover or
+  /// grab it, which is what a host wants for a marker it draws as a *readout*. An id that
+  /// names no cursor is ignored, like every other setter here.
+  void set_cursor_selectable(u32 id, bool selectable);
 
   /// The measurement for `id`. Re-evaluated only when a cursor moved, when the
   /// measure was added, or when the series underneath it changed: a different