deferredEvents lose output_index

Trap: tracking only unresolved candidate indices looks sufficient because deferred events flush together, but visible deferred items can reach the client without entering forwardedVisibleOutputIndices. A later hidden recall may compact an already-emitted source index and still execute recall. Fix: retain source-index metadata for every deferred indexed event and record each emitted visible index during flush; test unresolved ordinary tool → deferred sparse message → late recall, requiring one sanitized response.failed, output: [], and zero recall side effects.