Owner-monitored native SFTP/NBD duplex byte channel. No WebSocket framing.

A pull reader and bounded inbound queue expose backpressure; overflow is visible. Read EOF leaves the write direction usable over Unix; over TLS the peer's close_notify ends the connection, so later writes return :write_closed. Lifetime, chunks, byte totals and every blocking socket write are bounded. Closing the local channel never cancels an operation or owns host/Incus lifecycle.