Supervisor for HTTP listeners.

Listeners are started at runtime with caller-supplied config, so they cannot be static children of barrel_mcp_sup. They are added here instead, which gets them the two things a bare spawn never had: a restart when one crashes, and termination when the application stops rather than an orphan holding the port.

restart => transient, so a deliberate stop stays stopped while a crash is restarted. The restart intensity bounds a listener that cannot bind its port, which would otherwise loop.

Running without the application

The transports can also be driven with no barrel_mcp application around them, the same way barrel_mcp_http_engine:ensure_session_manager/0 copes with a missing supervisor. When this supervisor is not running, both functions below fall back to the unsupervised listener and the caller owns its lifecycle.