A Reactor.Middleware that bridges real Reactor step/run lifecycle events
onto :telemetry, so BRCE-adjacent receipt consumers can observe a
Reactor's execution without threading a receipt-writer through every step.
This middleware never writes a receipt itself -- it only emits real
:telemetry.execute/3 events carrying the real Reactor event, step name,
and context. Attaching a handler is the caller's job (see the Chicago test
for the canonical pattern: :telemetry.attach/4 sending to self()).