Per-session supervision tree for one FIX connection.

Supervises the session Client for a single {connection_id, session_name} pair, alongside any per-session subscriber processes the caller supplies via the :handlers option. The client starts first; the subscribers follow and register their interest with it. Under :rest_for_one, a subscriber that crashes restarts alone, while a client restart also restarts the subscribers after it — so they re-subscribe against the fresh session.

The session is named by :session_name (default :trading) and serves the roles given by :session_roles (default [:trading, :market_data]), which are the value of its registry entry — the source FixAlchemy.SessionDirectory resolves a role against.

:handlers is a list of modules (or child specs); each module receives the full connection opts. The generic engine ships no handlers — a broker adapter supplies its own (e.g. handshake, market-data, and portfolio subscribers).

The connection's dictionary is built before any child starts. A dictionary that cannot be built fails the start with that reason.