feat(multimodal): 视觉分层路由（多模态检测→主/视觉/OCR 工具）+ 语音 STT→TTS 降级链（异步）

internal/runtime/multimodal/voice/provider.go

@@ -0,0 +1,79 @@
+package voice
+
+import "context"
+
+// Provider is one rung in the voice degradation chain. It may implement zero,
+// one, or both directions; a nil return means that direction is not available
+// on this rung and the chain moves to the next.
+type Provider interface {
+	// Name returns a stable provider identifier.
+	Name() string
+	// STT returns the speech-to-text implementation, or nil when the provider
+	// cannot transcribe.
+	STT() STT
+	// TTS returns the text-to-speech implementation, or nil when the provider
+	// cannot synthesize.
+	TTS() TTS
+}
+
+// StaticProvider is a Provider with a fixed name whose STT/TTS are set at
+// construction. It is used by the concrete providers below.
+type StaticProvider struct {
+	name string
+	stt  STT
+	tts  TTS
+}
+
+// Name returns the provider identifier.
+func (p *StaticProvider) Name() string { return p.name }
+
+// STT returns the configured STT, or nil.
+func (p *StaticProvider) STT() STT { return p.stt }
+
+// TTS returns the configured TTS, or nil.
+func (p *StaticProvider) TTS() TTS { return p.tts }
+
+// NewProvider builds a Provider with the given name and optional directions it
+// supports.
+func NewProvider(name string, stt STT, tts TTS) *StaticProvider {
+	return &StaticProvider{name: name, stt: stt, tts: tts}
+}
+
+// CloudProvider is the cloud rung of the chain. It is an adaptation point: the
+// real STT/TTS implementations are wired in via NewProvider with cloud adapters.
+func CloudProvider(stt STT, tts TTS) *StaticProvider {
+	return NewProvider("cloud", stt, tts)
+}
+
+// LocalProvider is the local-model rung of the chain.
+func LocalProvider(stt STT, tts TTS) *StaticProvider {
+	return NewProvider("local", stt, tts)
+}
+
+// EdgeTTSProvider is the edge-tts rung (text-to-speech only).
+func EdgeTTSProvider(tts TTS) *StaticProvider {
+	return NewProvider("edge-tts", nil, tts)
+}
+
+// EspeakProvider is the espeak rung (text-to-speech only).
+func EspeakProvider(tts TTS) *StaticProvider {
+	return NewProvider("espeak", nil, tts)
+}
+
+// SilentProvider is the terminal rung. It always succeeds, returning an empty
+// transcription and empty audio, so a chain that includes it never errors.
+func SilentProvider() *StaticProvider {
+	return NewProvider("silent", silenceSTT{}, silenceTTS{})
+}
+
+// silenceSTT is the silent STT rung; it returns an empty transcription.
+type silenceSTT struct{}
+
+func (silenceSTT) STT(_ context.Context, _ Audio) (string, error) { return "", nil }
+
+// silenceTTS is the silent TTS rung; it returns empty audio.
+type silenceTTS struct{}
+
+func (silenceTTS) TTS(_ context.Context, _ string) (Audio, error) {
+	return Audio{}, nil
+}