feat(core-events): InMemoryEventBus with failFast option

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>

packages/core-events/src/in-memory-event-bus.test.ts

@@ -0,0 +1,49 @@
+import { describe, it, expect, vi } from "vitest";
+import { z } from "zod";
+import { defineEvent } from "@/event-descriptor";
+import { InMemoryEventBus } from "@/in-memory-event-bus";
+
+const evt = defineEvent("test.thing", z.object({ id: z.string() }).strict());
+
+describe("InMemoryEventBus", () => {
+  it("validates the payload via the descriptor's schema before fanout", async () => {
+    const bus = new InMemoryEventBus();
+    const handler = vi.fn();
+    bus.subscribe(evt, "test-consumer", handler);
+    await expect(bus.publish(evt, { id: 123 } as unknown as { id: string })).rejects.toThrow();
+    expect(handler).not.toHaveBeenCalled();
+  });
+
+  it("delivers to all registered handlers in parallel", async () => {
+    const bus = new InMemoryEventBus();
+    const a = vi.fn();
+    const b = vi.fn();
+    bus.subscribe(evt, "consumer-a", a);
+    bus.subscribe(evt, "consumer-b", b);
+    await bus.publish(evt, { id: "x" });
+    expect(a).toHaveBeenCalledWith({ id: "x" });
+    expect(b).toHaveBeenCalledWith({ id: "x" });
+  });
+
+  it("swallows handler errors by default (publisher's publish does not throw)", async () => {
+    const bus = new InMemoryEventBus();
+    bus.subscribe(evt, "boom", async () => {
+      throw new Error("subscriber blew up");
+    });
+    await expect(bus.publish(evt, { id: "x" })).resolves.toBeUndefined();
+  });
+
+  it("rethrows the first handler error when failFast is true", async () => {
+    const bus = new InMemoryEventBus({ failFast: true });
+    bus.subscribe(evt, "first", async () => {
+      throw new Error("first failure");
+    });
+    bus.subscribe(evt, "second", vi.fn());
+    await expect(bus.publish(evt, { id: "x" })).rejects.toThrow("first failure");
+  });
+
+  it("delivers nothing when no handlers are registered", async () => {
+    const bus = new InMemoryEventBus();
+    await expect(bus.publish(evt, { id: "x" })).resolves.toBeUndefined();
+  });
+});

packages/core-events/src/in-memory-event-bus.ts

@@ -0,0 +1,40 @@
+import type { z } from "zod";
+import type { EventDescriptor } from "./event-descriptor";
+import type { EventHandler, IEventBus } from "./event-bus.interface";
+
+export type InMemoryEventBusOptions = {
+  /** When true, rethrow the first handler error (default: false — errors swallowed). */
+  failFast?: boolean;
+};
+
+export class InMemoryEventBus implements IEventBus {
+  private readonly handlers = new Map<string, EventHandler<unknown>[]>();
+
+  constructor(private readonly options: InMemoryEventBusOptions = {}) {}
+
+  async publish<T>(
+    descriptor: EventDescriptor<string, z.ZodType<T>>,
+    payload: T,
+  ): Promise<void> {
+    descriptor.schema.parse(payload);
+    const subscribers = this.handlers.get(descriptor.name) ?? [];
+    if (subscribers.length === 0) return;
+    const settled = await Promise.allSettled(
+      subscribers.map((h) => h(payload)),
+    );
+    if (this.options.failFast) {
+      const failure = settled.find((s) => s.status === "rejected");
+      if (failure && failure.status === "rejected") throw failure.reason;
+    }
+  }
+
+  subscribe<T>(
+    descriptor: EventDescriptor<string, z.ZodType<T>>,
+    _consumerFeature: string,
+    handler: EventHandler<T>,
+  ): void {
+    const arr = this.handlers.get(descriptor.name) ?? [];
+    arr.push(handler as EventHandler<unknown>);
+    this.handlers.set(descriptor.name, arr);
+  }
+}