Add realtime rate chart to meldis

[jwnimmer-tri]

When a MeshcatVisualizer is connected directly to a Simulator, it can show a nice realtime rate chart (see #16981 for a screencap video).

We should enhance meldis to display the same chart, using the LCM message times to populate the rate.

[DamrongGuoy]

Thank you! Now I also understand the 73 rtr% means 73% realtime rate.

This feature would be very helpful when we try to speed up our simulators or evaluate new codes.

[jwnimmer-tri]

FYI a solution outline (untested):

--- a/bindings/pydrake/visualization/_meldis.py
+++ b/bindings/pydrake/visualization/_meldis.py
@@ -143,6 +143,13 @@ class _ViewerApplet:
             link_path = f"{self._path}/{robot_num}/{link_name}"
             pose = self._to_pose(message.position[i], message.quaternion[i])
             self._meshcat.SetTransform(path=link_path, X_ParentPath=pose)
+        sim_time = message.timestamp * 1e-6
+        wall_time = time.time()
+        if self._prior_times is not None:
+            sim_delta = sim_time - self._prior_times(0)
+            wall_delta = wall_time - self._prior_times(1)
+            self._meshcat.SetRealtimeRate(sim_delta / wall_delta)
+        self._prior_times = (sim_time, wall_time)
         if self._waiting_for_first_draw_message:
             self._waiting_for_first_draw_message = False
             self._build_links()