Add experimental in-process observation for Drizzle read models

[woksin]

Arc for TypeScript 0.38.0 adds experimental live observation for Drizzle SQL read models, the counterpart of Observe() on Entity Framework Core read models in Arc for .NET, limited to changes announced in the same process. npm publication remains disabled.

Added

• Opt-in, tenant-scoped observation for Drizzle read models: withDrizzle({ observation: DrizzleObservation.InProcess }).
  • DrizzleReadModels.observe(filter?) and observeById(key) return a live RxJS observable with current(), which model-bound observable queries can return and serve over SSE and WebSocket like any observable query.
  • observePage(filter, options) emits QueryPage results for low-level defineObservableQuery definitions; generated proxies don't support observable pages yet.
• DrizzleHandle.notifyChanged(tableOrReadModelType) announces a write. Inside an Arc command, notifications from the command and its nested commands are published once, when the outermost command finishes (also when it fails). Outside a command they are published immediately.
• The proxy generator recognizes query results whose class derives from an RxJS Observable or Subject, such as the observables returned by MongoDB and Drizzle read models, and asks for an Observable<...> annotation when it can't resolve the item type.

Changed

• rxjs is now a required peer dependency of @cratis/arc.drizzle.

Observation is experimental and deliberately narrow: only writes announced with notifyChanged in the same process and tenant are observed, pages are eventually consistent, a transaction the host holds around the whole Arc call must notify after it commits, and starting an observation without the opt-in fails instead of returning a stream that never updates. Cross-process change detection, such as PostgreSQL LISTEN/NOTIFY, is not included.

Refs #53