Solid writeup on the selector granularity, that's the part people miss when they first move off Context. One gap worth flagging: the persist middleware and Next.js SSR don't mix cleanly out of the box, the server has no localStorage so the first render always uses the default state, then the client rehydrates a second later. If the persisted cart count shows up in the UI, that's a visible flash or a hydration mismatch warning unless you gate the read behind a mounted check.