Multi-fd per handler reactor

[mrabine]

Description

The current EventHandler interface forces a one-to-one binding between a handler and a single file descriptor via the pure virtual handle() const noexcept getter. This design prevents a handler from registering multiple fds with the reactor.
This is a fundamental limitation.

Acceptance criteria

• EventHandler has no handle() method.
• onReceive, onClose, and onError receive the fd that triggered the epoll event.
• A single EventHandler instance can be registered for multiple fds via repeated addHandler call.