SQLite

The CLI and GUI are separate processes, and both write the same store.sqlite at the same time. It holds together because each write is wrapped whole in a single transaction that SQLite serializes, and rolls back whole on failure. Every change leaves one row in change_feed, and the GUI watcher re-reads only what changed by cursor and shows it.

Part 1 measured search running about twice as light, and Part 2 showed that the store is a graph. This part is how that graph is read: catching up on a task follows only the few connected records by index (O(result)). State like whether a task is startable or whether a decision is still current is never stored — it is derived on the spot from the edges, so the list, the card, and the start guard never disagree.