Simpler alternative to #17101. The bug here was that if an effect was scheduled during traversal, causing a follow-up traversal, an effect that was previously deferred would (having been marked CLEAN) incorrectly go into the #maybe_dirty_effects bucket rather than back into the #dirty_effects bucket. The solution is to persist the dirty bucket between runs.

Since block effects run immediately during traversal, they are always made CLEAN immediately, and so should be added to the #dirty_effects bucket straight away rather than being handled in a separate block_effects array — this allows us to simplify the effect target a little bit.

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