feat(eventbus): add EventBus interface and LocalEventBus implementation (#4229 phase A-B)

[OneStepAt4time]

Summary

Part of #4229 — Redis Streams EventBus + SSE bridge.

This PR delivers the MVP: the EventBus port interface and an in-process LocalEventBus implementation. Zero external deps.

New files

File	Lines	Purpose
src/event-bus.ts	62	EventBus interface + BusEvent type
src/local-event-bus.ts	108	In-process implementation with ring buffer replay
src/__tests__/local-event-bus.test.ts	142	11 tests

EventBus interface

interface EventBus {
  publish(channel, type, data): number;    // returns event ID
  subscribe(channel, handler): () => void;  // returns unsubscribe
  replaySince(channel, lastEventId): BusEvent[];
  destroy(): void;
}

Design decisions

• No Redis dependency — LocalEventBus is the default. Redis adapter is a future PR (phase C-D).
• Typed channels — session:abc123, global, etc. Implementations decide routing.
• Async delivery — setImmediate() for consistency with existing SessionEventBus.
• Ring buffer replay — configurable size, LRU eviction, cursor-based via lastEventId.

Cleanup included

• Removed stale existsSync/readFileSync imports from server.ts (arch(server.ts): God object — 1632 lines, 60 imports, 30+ module-level mutable globals #4243 step 7)

Stacking

• Stacks on refactor(server): extract ServiceContainer registration to boot/boot-services.ts (#4243 step 6) #4336 (boot-services extraction, merged)
• Future: Phase C (adapt SessionEventBus → EventBus), Phase D (Redis adapter, feature-flagged)

Verification

tsc --noEmit: clean
93 tests passed (11 new + 82 existing event tests)

Part of #4229. Closes phase A-B.

[aegis-gh-agent]

✅ Approved. Clean EventBus interface + LocalEventBus implementation.

event-bus.ts (62 lines): Minimal port interface — publish, subscribe, replaySince, destroy. Well-typed BusEvent with channel/id/type/timestamp/data. No over-engineering. ✅

local-event-bus.ts (104 lines): In-process implementation, zero external deps. Per-channel EventEmitters with ring buffers. Good: ID counter reset guard at MAX_SAFE_INTEGER, configurable buffer size, async delivery via setImmediate. ✅

Tests (11): Solid coverage — pub/sub, unsubscribe, multi-subscriber, channel isolation, replay, buffer eviction, monotonic IDs, destroy, cleanup on last unsub, timestamps. ✅

server.ts: Stale import cleanup (existsSync/readFileSync removed — the ones I flagged in #4336). ✅

Minor observations (not blocking):

1. void imm in publish() — either track the immediate handle for destroy() cleanup or just call setImmediate(...) without assignment.
2. Buffer eviction via splice(0, excess) is O(n) — fine at buffer=50, worth a circular buffer if it grows.
3. No guard for publish-after-destroy — calling publish() on a destroyed bus silently creates new emitters.

Approved-minor-bump label added, branch updated to re-trigger bump gate. CI: Node 20 failure is pre-existing E2E flake (Effort: high).