Add multiplexed ordered streams over KCP

Normalized profile

What this task asks for

Add a multiplexing layer that carries many ordered sub-streams over one KCP connection with flow control, priority scheduling, and SNMP counters. Multiplex independent ordered streams over one connection with client odd IDs and server even IDs. Apply per-stream byte windows so blocked writers resume on window updates without stalling other streams. Prioritize higher-priority stream data and send control frames before queued data frames. Expose stream lifecycle, deadlines, half-close behavior, and the six required SNMP counters. Unblock readers and writers with `io.ErrClosedPipe` when streams or sessions close. Make session close return promptly even when the underlying connection write is blocked.