Meshcat animations

[SeanCurtis-TRI]

What happened?

During the effort of implementing a solution to #19142, we became aware of a number of quirks with the animation mechanism.

The goal of this issue is as follows:

1. Enumerate the current behavior (at the time the issue was authored).
2. Determine if that behavior is intended/acceptable and if our documentation is sufficient to the behavior.
3. Determine if there's any future work we need to do to change the behavior/documentation.

Current behavior

TBD (Ran out of time today, I'll come back to fill this in later. This is a concrete placeholder to remind myself to capture this. Harass me if you don't see it.)

Version

No response

What operating system are you using?

No response

What installation option are you using?

No response

Relevant log output

No response

[jwnimmer-tri]

Harass me if you don't see it.

Ping.