only skip files that were already written when prerendering

[Rich-Harris]

fixes #4927

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[benmccann]

Do we want to add the fallback page to written?

Is it a little heavyweight to make a full integration test for this? Maybe we could get almost the same level of confidence from creating a write file helper method that both writes the file and adds it to written?

[Rich-Harris]

There's no value in adding the fallback page given that it's the last thing that happens and it's written unconditionally. Adding a new test app does feel like overkill but I'd be nervous to not have a test at all