build-computed

Create `ecs/computed-database/`: `computed-database.ts` (extends the previous layer, adds `computed` from `./computed/index.js`) plus a `computed/` folder — one `cached` derived value per file + barrel. Derivation logic lives in `data/`; a computed only wires a db observable to it. Comes after `transaction-database`. The how is in the auto-loading `features/ecs/computed.md` rule.