Reframe comments, docstrings, identifiers, changelog, and example
around a single explanation of the option: (1) turn strategies do not
consider user transcripts, letting the user turn end sooner, and (2)
the aggregator gathers user transcripts on its own after the turn
ends via a simple timer, then emits `on_user_turn_message_finalized`
with the new user context message.
The mechanism is generic, so internal aggregator vocabulary stays
generic ("transcript-gather", "after the user turn ends"); the
public-facing param docstring is the one place that explains the
"local turn detection drives a realtime service" use case. The stop
strategies' `wait_for_transcript` flag is pointed at as something
that's "usually flipped indirectly" by the aggregator param rather
than something to pair with it.
Renames internal state to match: `_expect_delayed_transcripts` →
`_aggregator_gathers_transcripts`, `_pending_finalization_*` →
`_transcript_gather_*`, `_finalize_delayed_user_message` →
`_finalize_user_message`, etc.
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