Behaviour for the pipe between a Mead.MCP.Connection and one MCP server.

A transport is a process started by, and linked to, its owner (the connection). It moves opaque binaries, one JSON message each, and knows nothing about MCP. It tells the owner what happened with plain messages:

• {:mcp_message, transport, binary} - one complete message arrived from the server.
• {:mcp_closed, transport, reason} - the pipe is gone and the transport is shutting down.
• {:mcp_send_failed, transport, id, reason} - the message sent with that id (or nil for a notification) could not be delivered or answered at the transport level.

A transport never raises to its owner for an expected failure. When the owner dies, the transport must release whatever it holds, including an operating-system process.