feat(voice): 语音silent降级信号(C2) — provider仅在结果为空时出现

- voice/Chain+Service: STTWithProvider/TTSWithProvider(真实档位cloud/local/edge-tts/espeak/silent), 旧方法改薄包装
- gateway: transcribe空文本带{text,provider} / voice_output空音频带provider / speech空音频tts_empty+X-Voice-Provider头
- middleware: CORS Access-Control-Expose-Headers: X-Voice-Provider
- 可选接口 voiceProviderReporter(type断言透传), 自定义实现回退silent
- 新增测试10个(provider_signal/voice_signal), build/vet/test全绿+交叉编译
- Docs/api-client v0.23 + api-reference CORS 同步

internal/services/gateway/middleware.go

@@ -25,6 +25,7 @@ func (s *Server) cors(next http.Handler) http.Handler {
 			w.Header().Set("Vary", "Origin")
 			w.Header().Set("Access-Control-Allow-Methods", "GET, POST, PUT, DELETE, OPTIONS")
 			w.Header().Set("Access-Control-Allow-Headers", "Authorization, Content-Type")
+			w.Header().Set("Access-Control-Expose-Headers", voiceProviderHeader)
 			w.Header().Set("Access-Control-Max-Age", "600")
 		}
 		if r.Method == http.MethodOptions {