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.

include/plot/plot/plot.hpp

@@ -229,6 +229,13 @@ class Plot {
   /// Measures the interval between two cursors over the series of `trace`.
   u32 add_measure(u32 cursor_a, u32 cursor_b, u32 trace = 0);
   u32 add_annotation(const Annotation& annotation);
+  /// Moves an annotation that is already in the plot, keeping its id -- what a host whose
+  /// annotation is a *readout* needs (the point where a cursor crosses a trace moves with
+  /// the cursor). `Plot` has no `remove_annotation`, so the same parking discipline as the
+  /// cursors applies: set `visible = false` rather than dropping it, and re-place where it
+  /// belongs. An id that names no annotation is ignored, not an error, exactly as every
+  /// other setter here treats a stale id.
+  void set_annotation(u32 id, const Annotation& annotation);
   /// Adds a region. Its bounds are ordered and, when the region asks to snap, pulled
   /// onto the sample grid as it is added -- exactly as `add_cursor()` places a
   /// snapping cursor -- so a region handed to a plot that already holds samples is on