feat(plot): set_annotation(), so a readout annotation can be moved

A host whose annotation is a readout -- the point where a cursor crosses a
trace -- has to move it every frame, and `add_annotation()` only appends:
the id would change every frame and the annotation count grow without bound.
The setter keeps the id, ignores a stale one exactly as the cursor setters
do, and pairs with parking (`visible = false`) since there is no
`remove_annotation`.

Tested by `an_annotation_can_be_moved_by_id` in tests/test_plot.cpp.

src/plot/plot.cpp

@@ -306,6 +306,11 @@ u32 Plot::add_annotation(const Annotation& annotation) {
   return static_cast<u32>(annotations_.size() - 1);
 }
 
+void Plot::set_annotation(u32 id, const Annotation& annotation) {
+  if (id >= annotations_.size()) return;
+  annotations_[id] = annotation;
+}
+
 u32 Plot::add_region(const Region& region) {
   // A region whose bounds are not numbers is not an interval this plot can hold, and a
   // `Free` one stored that way could never be dragged back (every motion re-derives both