Add reactor-backed proactor implementation

[mrabine]

Description

Introduce a Proactor class that emulates asynchronous I/O on top of the existing epoll Reactor. Each submitted IoOperation registers the proactor as an EventHandler on the operation's fd; when epoll signals readiness the proactor executes the syscall and signals completion. The implementation is intentionally forward-compatible with a future io_uring backend (JOIN_HAS_IO_URING): IoOperation opcodes, lifecycle states, and the cancel API are designed to map directly onto io_uring semantics when that backend is introduced.
A ProactorThread convenience class mirrors ReactorThread and owns a Proactor running on a dedicated background thread.

Acceptance Criteria

• IoOperation struct is introduced with opcodes Read, Write, ReadFixed, WriteFixed, RecvMsg, SendMsg, Accept and lifecycle states Idle, Submitted, Cancelling
• Factory methods makeRead, makeReadFixed, makeWrite, makeWriteFixed, makeRecvmsg, makeSendmsg, makeAccept are implemented
• ReadFixed and WriteFixed degrade silently to Read and Write on the reactor-backed proactor
• Proactor::submit registers the fd with the reactor and transitions the op to Submitted
• Proactor::cancel removes the fd from the reactor and transitions the op back to Idle
• Proactor owns its Reactor instance
• Proactor::run and Proactor::stop delegate to the owned Reactor
• ProactorThread owns a Proactor and a background Thread, mirrors ReactorThread API (affinity, priority, handle, mlock)