TDD: red (test-only), then green. InMemoryEventBus validates payloads via
descriptor.schema before fanout, delivers to all handlers via Promise.allSettled,
swallows handler errors by default, and rethrows first error when failFast:true.
5 new tests (8 total passing).
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
