Add adapter-backed transactions

[KidkArolis]

What changed

• add an optional atomic transaction capability to the adapter contract
• add a schema-typed figbird.transaction(tx => ...) CRUD collector with optimistic and confirmed operations
• coordinate transaction dispatch with Figbird's per-record mutation lanes, grouping optimistic projection, commit, and rollback notifications across services
• add feathersBatchTransactions() for the existing api/batch tuple/result contract
• document the API, constraints, and Feathers setup; export the new public types and helpers

Why

Mutation queues preserve ordering but do not provide server atomicity. Multi-service features such as moving a card while updating column counters need an explicit all-or-nothing backend capability. This API only exists when the adapter supplies that guarantee and never labels sequential requests as a transaction.

User impact

Apps with an atomic batch endpoint can express multi-service writes through the same schema-typed Figbird DSL. Optimistic changes appear together and roll back together on failure. Existing adapters and mutation APIs are unchanged; transaction support is opt-in.

Validation

• npm run tsc
• npm run lint
• npm run ava — 349 tests passed
• npm run test — full type, lint, format, coverage, and test suite passed
• npm run build