Function enter_emit_envelope 

pub fn enter_emit_envelope(observer: &Arc<dyn Observer>, env: ObsEnvelope)

Dispatch one envelope through the observer with the re-entry guard held. Spec 11 § 3.1 “Re-entry and the CAN_ENTER cell”.

All emit paths (Emit::emit, the obs::emit! macro, the <EventBuilder>::emit setter) route through this so a sink that synthesises a new envelope from inside Observer::emit_envelope sees observer() returning NoopObserver and the inner emit becomes a no-op.