fix: prevent batches from getting intertwined

[Rich-Harris]

No test for this because I can't figure how to make something broken without it, but I'm fairly sure this is necessary. While debugging an app that uses remote functions, I noticed that batches weren't reliably getting removed, and in at least some cases the root cause is that the queueMicrotask callback in a boundary is getting tangled up with the queueMicrotask callback in Batch.ensure

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