A completion-only speech ledger cannot prove a contiguous delivered prefix

A completion-only speech ledger cannot prove a contiguous delivered prefix. If an earlier synthesis request is cancelled before producing audio, a later successful request may appear to be the first completed item even though the intended response has a gap. Record ordered starts for every attempt, retain incomplete attempts, and require a contiguous sequence of eligible playback receipts before calling anything a prefix. Keep individually confirmed later units distinct from a confirmed prefix. This applies to streamed speech, media chunks and other pipelines where completion events omit failed attempts; it does not prove receiver comprehension.