Switch the vocabulary for the timer-driven phase that runs when
`wait_for_transcript_to_end_user_turn=False`. "Transcript gather" was
too vague to be self-documenting; "post-turn transcript wait" names
when it happens (after the user turn ends) and what it's for (waiting
for late-arriving transcripts).
Renames the internal property to `_wait_for_post_turn_transcripts`
and the supporting state/method names to match
(`_post_turn_transcript_wait_task`, `_complete_post_turn_transcript_wait`,
etc.). Updates docstrings, comments, log messages, the example
inline doc, and the test prose to use the new vocabulary consistently.
ea96b7aec7