build-computed

Legacy feature (has `data/state/` or `services/main-service/`)? Extend it in place per the legacy rules; don't migrate unless asked. Skip if the spec has no derivations and the UI needs no derived values. Create `ecs/computed/`: `computed-database.ts` (extends the previous layer, adds `computed` from `./index.js`), one `cached` derived value per file, and a barrel. Wire `data/values/` helpers to the minimal observables. Name each entity-id-list computed in the implementation's `hydrate`, and pass `computedPlugin: ComputedDatabase.plugin`. Gate: each derivation's computed conforms. The how is in the auto-loading `features/ecs/computed.md` rule.