DI registrations create separate backing stores when tracing is disabled #610

Related test issue

Discovered while implementing #608 — event-store dependency injection coverage.

Problem

With EventuousDiagnostics disabled, store registration methods register implementations or factories separately for each interface. Each interface therefore gets its own singleton rather than sharing one backing instance.

A successful append through IEventWriter can be followed by StreamNotFound when the same stream is read through IEventReader. This is reproducible with the stateful InMemoryEventStore; it is not a claim that persistent providers lose stored data. Factories can also run more than once, and standalone reader/writer registrations can ignore an already registered concrete store.

Affected APIs include the generic and factory overloads of AddEventStore<T>, AddEventReaderWriter<T>, and separately composed AddEventReader<T> / AddEventWriter<T>.

Reproduction

1. Disable diagnostics, preserving the previous setting for restoration afterward.
2. Build a host with services.AddEventStore<InMemoryEventStore>().
3. Resolve IEventWriter and IEventReader from that host.
4. Append an event to a new stream through the writer.
5. Read that stream through the reader.

Actual: the read throws StreamNotFound because the interfaces resolve to different stores.

Expected: all interfaces registered for the same implementation share one backing singleton. Changing tracing configuration must not change which events are visible.

Fix and acceptance criteria

• Register the concrete implementation or factory once, then resolve interface aliases from that singleton.
• Verify append/read behavior through a real host for store, combined reader/writer, and separately registered reader/writer APIs.
• Cover generic and factory overloads with tracing enabled and disabled.
• Factories are invoked once for the shared instance, and interface resolutions remain singleton across scopes.
• Standalone reader/writer registrations reuse a pre-registered concrete instance without invoking a replacement factory.
• Tests restore the process-wide diagnostics setting and cannot race with other tests that observe it.

Existing regression tests and implementation

StoreRegistrationTests.cs in src/Extensions/test/Eventuous.Tests.DependencyInjection contains:

• WrittenEventsAreVisibleThroughEveryRegisteredInterface
• StandaloneRegistrationUsesTheExistingBackingStore
• The tracing-disabled cases of RepeatedRegistrationKeepsOneServiceAndOneFactory

These scenarios failed before the production fix. The complete DI project passes all 31 tests on .NET 10 after the fixes for this issue and the companion registration issue.

An implementation is already committed locally on fix/issue-608-store-registrations, commit df3dbe48. It also fixes duplicate registrations and overridden services; this draft does not imply the fix has been published or merged.