Tag metadata tokens as {\"__meta\"} to separate TTS from progress messages
stream_response() now yields plain str for actual answer tokens and
{\"__meta\": str} dicts for progress/thinking/source metadata.
Consumers (WebUI, Telegram) can filter __meta tokens for TTS/display.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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