Allow passing pre-computed phonemes to Kokoro TTS

[yocontra]

Right now if you want to use Kokoro TTS, you have to go through the built-in phonemis G2P pipeline. There's no way around it. This PR adds generateFromPhonemes / streamFromPhonemes methods that let you skip phonemis and pass your own IPA phoneme strings directly to the synthesis engine.

Why would you want this? A few reasons we've run into:

• phonemis doesn't handle every word well. Libraries like phonemizer (espeak-ng backend) do better on edge cases, foreign words, etc.
• Custom lexicons. If you have domain-specific pronunciation (game character names, medical terms), you probably want control over the G2P step.
• Server-side G2P. Pre-compute phonemes on a server with a proper NLP pipeline, send them to the device.
• Languages phonemis doesn't cover yet.

What changed

The existing generate() / stream() methods now delegate to shared internal helpers (generateFromPhonemesImpl / streamFromPhonemesImpl). The new public methods call the same helpers but skip the phonemizer_.process() step. No behavior change for existing callers.

Changes across layers:

• C++ Kokoro: generateFromPhonemes, streamFromPhonemes + input validation (empty string, invalid UTF-8)
• JSI ModelHostObject: exposes new methods
• TextToSpeechModule: forwardFromPhonemes(), streamFromPhonemes() (shared streamImpl helper, no copy-paste)
• useTextToSpeech hook: same, with shared guard + streaming orchestration
• Types: TextToSpeechPhonemeInput, TextToSpeechStreamingPhonemeInput, TextToSpeechStreamingCallbacks

Usage

const tts = new TextToSpeechModule();
await tts.load(config);

// text path (unchanged -- goes through phonemis)
const audio = await tts.forward("Hello world");

// phoneme path (bypasses phonemis)
const audio = await tts.forwardFromPhonemes("həloʊ wɝːld");

// streaming
for await (const chunk of tts.streamFromPhonemes({ phonemes: "həloʊ wɝːld", speed: 1.0 })) {
  playAudio(chunk);
}

Test plan

• Existing generate() and stream() still work (refactor is internal)
• generateFromPhonemes() with known Kokoro IPA strings
• streamFromPhonemes() produces same audio as stream() for identical phonemes
• Multi-byte UTF-8 phoneme characters (ʊ, ɪ, ŋ, etc.)
• Empty string and invalid UTF-8 rejected with proper error

[IgorSwat]

Hey @yocontra !

Thank you very much for your contribution. I reviewed the code and it looks pretty good - and the new API methods indeed make a lot of sense.
I just added some changes to the docs to match the new API.

I can also reveal that we have started to work on big update to both Phonemis phonemizer as well as Kokoro-based core, which will add new languages support and improve the performance of the model significantly. So if you are interested in this module, stay tuned for that :)