feat: epoll EventLoopProvider implementation (driver FD registration)

[FumingPower3925]

Summary

Implement the EventLoopProvider and WorkerLoop interfaces on the epoll engine, allowing database/cache driver connections to be registered on epoll workers.

Design

Changes to engine/epoll/loop.go

• Add driverConns map[int]*driverConn to Loop struct
• driverConn holds onRecv func([]byte), onClose func(), and a write buffer
• RegisterConn(fd): adds FD to epoll interest set with EPOLLIN | EPOLLET, stores in driverConns
• UnregisterConn(fd): removes from epoll, calls onClose, deletes from map
• Write(fd, data): queues data and adds EPOLLOUT to interest set
• In the event loop dispatch: check if FD is in driverConns before HTTP conn lookup — call onRecv for EPOLLIN, flush write buffer for EPOLLOUT

Atomic Guard

• hasDriverConns atomic.Bool on Loop — set to true on first RegisterConn, checked before the driver FD lookup path to avoid map access when no drivers are registered

Acceptance Criteria

• Epoll Loop implements EventLoopProvider
• RegisterConn / UnregisterConn / Write work correctly
• Driver FD events are dispatched to callbacks
• Zero overhead when no driver connections are registered (atomic guard)
• Unit tests for registration, data receive, write, and unregistration

Dependencies

• Depends on 113