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.

tests/test_plot.cpp

@@ -2584,3 +2584,37 @@ PLOT_TEST(xy_series_measurement_uses_the_true_nearest_samples) {
   PLOT_CHECK_NEAR(stats.min, 10.0, 1e-9);
   PLOT_CHECK_NEAR(stats.max, 10.0, 1e-9);
 }
+
+// A host whose annotation is a readout -- the point where a cursor crosses a trace --
+// moves it every frame. The id has to survive that, and a stale id must be ignored
+// rather than read or written out of bounds, exactly as the cursors' setters behave.
+PLOT_TEST(an_annotation_can_be_moved_by_id) {
+  plot::Plot p;
+  const plot::Annotation pin{1.0, 2.0, "first", plot::Color::hex(0xE8E8E8), 0u, 0u, true};
+  const u32 id = p.add_annotation(pin);
+  PLOT_CHECK(p.annotation(id).x == 1.0);
+  PLOT_CHECK(p.annotation(id).text == std::string("first"));
+
+  plot::Annotation moved = pin;
+  moved.x = 3.5;
+  moved.y = -1.25;
+  moved.text = "moved";
+  p.set_annotation(id, moved);
+  PLOT_CHECK_NEAR(p.annotation(id).x, 3.5, 1e-12);
+  PLOT_CHECK_NEAR(p.annotation(id).y, -1.25, 1e-12);
+  PLOT_CHECK(p.annotation(id).text == std::string("moved"));
+  PLOT_CHECK(p.annotation_count() == 1u);  // moved, not added
+
+  // Parked rather than removed, like a cursor: the id stays valid and reads back hidden.
+  plot::Annotation parked = moved;
+  parked.visible = false;
+  p.set_annotation(id, parked);
+  PLOT_CHECK(!p.annotation(id).visible);
+  PLOT_CHECK(p.annotation_count() == 1u);
+
+  // A stale id changes nothing (and reads a default, as the accessors promise).
+  plot::Annotation other = pin;
+  other.x = 99.0;
+  p.set_annotation(42u, other);
+  PLOT_CHECK_NEAR(p.annotation(id).x, 3.5, 1e-12);
+}