arch: migrate EventBus tests from layer() to per-test Layer.build

[cursor]

Problem

.agents/rules/stack.md requires per-test Layer.build inside it.effect bodies. Using @effect/vitest layer(...) memoizes the Layer graph across tests in a suite, which can leak stateful services between tests.

Current violations:

• packages/server/test/events.test.ts — layer(EventBusLayer)("EventBus", ...)
• packages/server/test/harness/session.test.ts — module-scoped const layer = SessionServiceLayer.pipe(Layer.provide(EventBusLayer)) with a shared run() helper

packages/server/test/observability/turn-log.test.ts already documents the correct pattern with an inline comment explaining why layer(EventBusLayer) is wrong.

Why it matters

Stateful services (EventBus, session instances) can leak between tests, producing order-dependent failures that are hard to reproduce.

Suggested fix

Migrate these suites to @effect/vitest it.effect with Layer.build (or scoped Effect.gen) per test, matching turn-log.test.ts and launcher.test.ts.

Related

• Protect shared stateful Effect Layers at the composition root #159 (Protect shared stateful Effect Layers at the composition root)

Severity

P2