Release v19.50.0

Fixed

• Reactor and reducer dispatch caches are isolated to each client's event registry, so registrations from one client cannot change which events another client handles. Registry changes invalidate cached dispatch maps. (part of #4732)
• Event dispatch includes exact registry-known event types without an [EventType] attribute, while retaining base/interface dispatch and historical-generation exclusions. (part of #4732)
• Reactor delivery identities, event-sequence inference and implicit external-store subscriptions consistently use registered event identities and store metadata. Private helpers referencing events outside a filtered registry no longer cause observer discovery to fail. (part of #4732)
• Event-target read-model registration and child event-property mapping resolve event identities through the client registry, preserving existing publication checks and identity defaults. (part of #4732)