tickSource

What the last heartbeat handed in — the MinecraftServer on the server lane, the Minecraft on client/render; null before the first pump. Checked every pump because isReady and run_command read it from the HTTP thread, and both must get a current value on an idle lane too.

Held weakly: a stopped integrated server would otherwise stay pinned, whole world included, until the next one pumps. A running source is always strongly reachable — its own thread is running it.