Fix area visualizer breaking after stop/start cycle

[Copilot]

stopVisualizing() was cancelling all child jobs in scope, including the long-lived consumeEach worker that drains computationChannel. Since startVisualizing() is delegated and never recreates the worker, any settings/corner mutations after the first stop/start cycle silently enqueued to a dead channel.

Changes

• Store worker reference — private val workerJob: Job holds the consumeEach coroutine launched in init
• Preserve worker in stopVisualizing() — only cancels non-worker scope children:

  scope.coroutineContext[Job]?.children?.forEach { child ->
      if (child !== workerJob) child.cancel()
  }

• Guard recompute() against stale execution — added !isVisualizing() early-returns after each suspension point to prevent an in-flight recompute from spawning entities after the visualizer has been stopped

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[Copilot]

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Warning

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• piston-data.mojang.com
  • Triggering command: /usr/lib/jvm/temurin-17-jdk-amd64/bin/java /usr/lib/jvm/temurin-17-jdk-amd64/bin/java --add-opens=java.base/java.lang=ALL-UNNAMED --add-opens=java.base/java.lang.invoke=ALL-UNNAMED --add-opens=java.base/java.util=ALL-UNNAMED --add-opens=java.prefs/java.util.prefs=ALL-UNNAMED --add-exports=jdk.compiler/com.sun.tools.javac.api=ALL-UNNAMED --add-exports=jdk.compiler/com.sun.tools.javac.util=ALL-UNNAMED --add-opens=java.base/java.util=ALL-UNNAMED --add-opens=java.prefs/java.util.prefs=ALL-UNNAMED --add-opens=java.base/java.nio.charset=ALL-UNNAMED --add-opens=java.base/java.net=ALL-UNNAMED --add-opens=java.base/java.util.concurrent=ALL-UNNAMED --add-opens=java.base/java.util.concurrent.atomic=ALL-UNNAMED --add-opens=java.xml/javax.xml.namespace=ALL-UNNAMED --add-opens=java.base/java.time=ALL-UNNAMED -Xmx4G -Dfile.encoding=UTF-8 -Duser.country -Duser.language=en -Duser.variant (dns block)

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