Host-supervised singleton runtime for EtherCAT.

Start this supervisor under the host application's supervision tree before calling EtherCAT.start/1. It owns the singleton registries, dynamic supervisors, and master process used by the public EtherCAT API.

EtherCAT does not start itself as an OTP application. The supported model is for the host application to supervise EtherCAT.Runtime directly, either statically or via DynamicSupervisor.start_child/2.

Example:

children = [
  {EtherCAT.Runtime, []}
]

{:ok, _supervisor} = Supervisor.start_link(children, strategy: :one_for_one)

{:ok, session} = EtherCAT.start(backend: {:raw, %{interface: "eth0"}})
:ok = EtherCAT.await_ready(session)
{:ok, :preop_ready} = EtherCAT.state(session)
:ok = EtherCAT.stop(session)

This discovery-only example requires raw-socket privileges and a dedicated EtherCAT interface. It holds slaves in PREOP; cyclic I/O needs explicit slave and domain configuration. See EtherCAT.Simulator for a no-hardware example. In an application, add the child to your existing supervisor instead of starting a second supervision tree. Stopping a session leaves the host-owned runtime running.

Only one EtherCAT runtime may be active per BEAM node. The master and its supporting registries keep their singleton process names. Each start returns an opaque EtherCAT.Session required by all public operations, so long-lived callers cannot accidentally cross a stop/start boundary.