Arc framework runtime.
Holds the reusable web/runtime machinery — Actix middleware, Tera helpers,
the event-sourced stack wiring, the websocket transport, and the server
bootstrap — behind a builder seam. Applications depend on this crate by
version and plug their own aggregate, projectors, and routes in through
[ArcApp::builder]. No concrete domain type lives here: the runtime is
generic over [arc_core::aggregate::Aggregate].