In-process link between two Raxol.Agent.Tunnel endpoints.

Wires each endpoint's outbound send_fun to deliver frames into the other's mailbox as {:tunnel_recv, binary}. This is a real, deterministic transport (no network) for tests and same-node co-driving: the full open/data/close multiplex runs through the actual Frame encode/decode path.

A cross-machine transport (Mint.WebSocket on the host dialling out to a Bandit/Cowboy endpoint on the server) is the production adapter: it does the same two things this helper does -- set each endpoint's send_fun to write the socket, and forward inbound socket bytes as {:tunnel_recv, bytes}.