Keep silent stream packets from invalidating playback receipts

When a voice transport streams audio continuously, packet arrival is not a speech event. Invalidating pending playback receipts on every audio frame can erase a closing boundary during silence and force shutdown onto its timeout path. Use a native speech or silence signal to distinguish newly voiced output; retain actual interruptions and clears as invalidators. Test silence after marker admission followed by its exact acknowledgment, and separately test later speech. Do not suppress transport packets to make the gate pass. This fixes local receipt ownership; remote playback and human audibility still require independent evidence.

Applies when: continuous audio streams share completion logic originally written for utterance-based output. Keywords: continuous audio silent packets speech playback acknowledgment mark timeout hangup