// Package provider implements talk's TTS providers and their fallback
// chain. The contract (Name/Available/Synthesize) carries the four-tier
// chain: ElevenLabs → OpenAI TTS → Edge TTS → OS built-in (overridable via
// --provider/config provider). The cloud providers return MP3 (keys come
// exclusively from TALK_ELEVENLABS_KEY and TALK_OPENAI_KEY, endpoints are
// configurable and therefore fakeable); osbuiltin returns RIFF/WAVE PCM.
// The documented optional extension DirectSpeaker covers platforms whose
// OS TTS cannot provide audio bytes (Linux speech-dispatcher); Chain.Speak
// handles that case, including silent discarding on the null sink, so that
// the CLI and the daemon use the same chain unchanged.
package provider