feat(core-events): IEventBus interface + symbol registry

IEventBus defines publish<T> and subscribe<T> with consumerFeature parameter.
CORE_EVENTS_SYMBOLS.IEventBus uses Symbol.for for cross-module identity.
Typecheck passes.

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

packages/core-events/src/symbols.ts

@@ -0,0 +1,3 @@
+export const CORE_EVENTS_SYMBOLS = {
+  IEventBus: Symbol.for("@repo/core-events/IEventBus"),
+} as const;