feat(core-events): scaffold events optional core

Scaffolded via pnpm turbo gen core-package events (generator-first,
ADR-022 pre-shipped zod trace landed in docs/library-decisions/).
Generator wired transpilePackages + E1/J eslint rule block. Two
story-00-precedent additions so the coverage gates pass: the
@vitest/coverage-v8 devDep (L0 sweep) and the honest symbols.ts
exclude (mirroring core-shared/vitest.config.ts) plus its missing
ALLOWED_GLOBS mirror in scripts/coverage/diff.mjs (L1 gate). No
consumers wired — compile-green floor for the walking-skeleton
PRD (ADR-027).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016j8z4VHjedXDTjEDNg7qHK

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

@@ -0,0 +1,42 @@
+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");
+      // Only the first rejection is rethrown. Other failures are intentionally
+      // dropped — `failFast` is a test-affordance, not a fault-tolerance design.
+      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);
+  }
+}