Does the note cache grow forever? Does it get cleaned up?
It's cleaned up automatically, so it won't grow without bound. The app garbage-collects its internal note cache (roughly every 10 minutes during syncing) and removes two kinds of entries: "orphaned" ones (notes no longer linked by any current to-do) and "aged" ones (notes you haven't opened in over 30 days). Both can be re-fetched from your device whenever you need them again. This cleanup is strictly limited to the app's own cache folder; it NEVER touches the PDFs you've saved in ~/Documents/Supernote.