Subsystem

Smallest lifecycle interface: start / stop.

template <typename T>
concept Subsystem = requires(T s) {
  s.start();   // initialise resources, spawn threads, etc.
  s.stop();    // graceful shutdown
};

Trait & Handle

Piece	Responsibility
SubsystemTrait	Builds a static v-table with start and stop via meta::wrap.
SubsystemRef	Two-pointer handle {void*, VTable*}; calls into the v-table with zero virtuals.

SubsystemRef API

void start() const;
void stop()  const;

Purpose

Provide a common lifecycle contract so the Engine can orchestrate diverse components (buses, strategies, sinks, risk managers) in a deterministic order.

Zero overhead: one pointer indirection per call.