Thoroughly test multiplayer

[Indy2222]

• Identify and plan development of necessary tooling for debugging & diagnostics.
• Ensure there are now undesired disconnects or crashes.
• Ensure there are now de-syncs.
• Make sure moving units don't flicker / jump around.
• Ensure that bandwidth usage stays reasonable.

[github-actions]

There is no activity on the issue for some time.