Add an `LLMMarkerFrame(DataFrame)` for sideband LLM markers that need
to be persisted to context but should not flow through the standard
text path (TTS, transcript). The frame carries an
`append_to_context_immediately` flag so the assistant aggregator can
either commit the marker as a stand-alone message (○ / ◐) or merge it
with the upcoming aggregation as a prefix on the response (✓).
`UserTurnCompletionLLMServiceMixin` now emits `LLMMarkerFrame` instead
of pushing the marker as `LLMTextFrame(skip_tts=True)`, which fixes
the case where an incomplete-turn marker (○ / ◐) was aggregated by
the assistant aggregator but never committed to the context because
the assistant turn lifecycle didn't run to completion (no spoken
response, no `LLMFullResponseEndFrame`-driven `push_aggregation`).
The frame is intentionally generic so other components — STT services
with built-in turn signals, end-of-turn classifiers, custom
annotations — can use the same mechanism to inject sideband signals
into the assistant context.
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