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/event-bus.interface.ts

@@ -0,0 +1,23 @@
+import type { z } from "zod";
+import type { EventDescriptor } from "./event-descriptor";
+
+export type EventHandler<T> = (event: T) => Promise<void>;
+
+export interface IEventBus {
+  publish<T>(
+    descriptor: EventDescriptor<string, z.ZodType<T>>,
+    payload: T,
+  ): Promise<void>;
+
+  /**
+   * Subscribe a handler. `consumerFeature` is the kebab-case name of the
+   * subscribing feature (e.g., "marketing-pages"). InMemoryEventBus uses it
+   * only as a debug tag; PayloadJobsEventBus uses it to name the fan-out task
+   * slug deterministically (`__events.<event>.<consumerFeature>`).
+   */
+  subscribe<T>(
+    descriptor: EventDescriptor<string, z.ZodType<T>>,
+    consumerFeature: string,
+    handler: EventHandler<T>,
+  ): void;
+}

packages/core-events/src/symbols.ts

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