Voice nodes (openpanel V4): stt, tts, wren reply/unhandled, socket publish

- `stt`: one realtime transcription session per utterance on the speech
  server's url, opened at `listening` true or the first chunk, committed at
  false; a whole clip goes to /v1/audio/transcriptions. Returns at once and
  publishes `partial` and `text` when the server answers.
- `tts`: one /v1/audio/speech call per sentence, cached by voice and text in
  node state; a reference is marked `append` while earlier ones still play.
- wren: `reply` says what was applied, `unhandled` carries the prompt when
  nothing was. The install hint names the git pin (PyPI's `wren` is another
  package).
- ai/client: transcribe, speak and transcribe_stream beside stream_completion.
- A device socket publishes scalar `in` ports as {"type": "publish"} text,
  in order with its binary frames.
- Pipeline.publish walks only what its values reach: a node downstream of a
  consumer that published nothing no longer runs again (wren re-acted on
  every microphone chunk).

docs/code/nodes.md

@@ -179,7 +179,10 @@ and ARM, so nothing has to be installed into the image.
 
 openpanel publishes a 16 kHz mono WAV on its `mic` port after its wake word,
 and plays any `audio` reference sent to its `speaker` port (WAV, MP3 or AAC).
-Two Function nodes close that loop with local models.
+The [Speech to text](../reference/node-types.md#speech-to-text) and
+[Text to speech](../reference/node-types.md#text-to-speech) nodes close that
+loop through a speech server on a GPU. Without one, two Function nodes close it
+with local models.
 [faster-whisper](https://github.com/SYSTRAN/faster-whisper) turns speech into
 text and [piper](https://github.com/OHF-Voice/piper1-gpl) turns text into
 speech.