Hub and Companion

TODO: this page will describe the two-component architecture. The Hub is the macOS app that holds the data, runs the local model, and does the heavy lifting. The Companion is the iOS app that talks to the Hub over Wi-Fi for capture, search, and notifications.

Cover: what each component owns, what state lives where, how they sync, and what happens when one is offline.

Owner: @andy. Cross-reference: the v3 architecture description in the project README, CM031 (Companion), CM050 (Hub update system).