fix(events): edge bus Subscribe 拆锁——磁盘回放移出 b.mu 临界区

[DeliciousBuding]

范围（#2154 性能 lane F15，P1）

Bus.Subscribe 原先在 b.mu 内调用 EventLog.ReadFrom（文件 IO + 逐行 unmarshal，日志最大 ~50MiB）：一个带旧 cursor 的重连客户端可把整个 edge 事件的 Publish fanout 停住整个回放时长。

• ReadFrom 移到取锁之前执行（eventLog 指针在构造期固定，竞态安全）
• b.mu 内只保留 subscriber 注册 + 内存 history 回放 + 合并（纯内存）
• 合并逻辑抽成纯函数 mergeReplayWithLog（gap 注入 / seenSeqs 去重 history 优先 / seq 排序），新增 3 个契约测试
• 回放语义不变：并发发布由 history 去重覆盖；gap/排序与旧实现一致

验证

• go test ./edge-server/... -short -count=1 全绿
• go vet ./internal/events/ 绿；新增 3 测试绿
• git diff --check 干净

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