docs(05): research event bus infrastructure phase

Phase 5: Event Bus Infrastructure
- Standard stack identified (MassTransit 9.0.0 inbox/outbox, retry, circuit breaker)
- Architecture patterns documented (global middleware, per-consumer overrides, DLQ routing)
- Pitfalls catalogued (middleware ordering, emulator DLQ, detection window)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: baotoq