Independent-review findings on the plugin PCM streaming path:
- stream_sample_rate was checked, then re-read and int()-ed in the
adapter: NaN/inf raised out of resolve_streaming_provider (the speaker
then played nothing) and 0.5 became sample_rate=0. Validate once
(finite, >= 1 Hz) and hand that int to _PluginPCMStreamer.
- `nous` is an alias of the openai path in _get_provider but not a
reserved name, so a pinned `tts.streaming.provider: nous` could stream
a plugin registered as `nous` while the sync path speaks OpenAI.
- Document that stream() must be thread-safe: the speaker prefetches up
to 3 sentences, unlike the serialized synthesize() path.
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