Every decision paging needs, as pure functions: which page a URL is asking for, which page is actually showable given how much there is, which slice of the items that is, and what page size to use. Pure because that is the only thing this project can unit-test — vitest runs in a node environment with no jsdom and no testing-library, so a hook or a component is only reachable through Playwright. Keeping the rules here means the rules have tests and the React wrapper stays thin enough not to need any, which is the same split filters.ts already uses for the URL.
An unrecognised page size is refused rather than clamped. A stored or hand-edited 5000 would render the entire catalogue in one page, which is the exact failure this issue exists to prevent, and clamping would quietly honour a value nobody offered. Storage access is guarded on both sides because localStorage is absent when there is no window and throws outright in some privacy modes, and neither is a reason for a customer to lose the catalogue — the worst acceptable outcome of a broken preference is the default.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
