Shutting down no longer crashes when custom types are in use

When a mesh shut down, the memory for each dynamically compiled NodeType was reclaimed a moment too early — while background rendering work that used those very types could still be running. Most of the time nothing noticed, but under load the process could abort outright instead of exiting cleanly, which showed up as unexplained crashes at the end of a test run or a pod restart.

The reclaim now happens at the true end of shutdown, after all in-flight work has been stopped and joined. Nothing is held any longer than before during normal operation: a single node going away while the mesh keeps running still releases its memory immediately.