Skip if this feature doesn't contain or require computed values.
Create services/main-service/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/services/main-service/computed.md rule.