fix: ensure that CSS is generated for the final frame of a transition

[Rich-Harris]

fixes #11081. No test because it doesn't reproduce in our test suite — I added a comment instead that is hopefully clear enough to prevent careless regressions

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[changeset-bot]

🦋 Changeset detected

Latest commit: 9439e7f

The changes in this PR will be included in the next version bump.

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[Rich-Harris]

self-merging since this is a fairly trivial change