Clarify racy array use in `TimerHeap` on scala-native

[durban]

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Description: https://github.com/scala-native/scala-native/issues/4288#issuecomment-2780673578

tl;dr: I didn't find explicit scala-native documentation about multithreaded use of arrays. The generated LLVM bitcode seems to contain non-atomic loads when reading length of elements. I think that might be undefined behavior (if we use it in a way we do in TimerHeap).