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/errors.go

@@ -0,0 +1,18 @@
+package voice
+
+import "errors"
+
+// Sentinel errors returned by the voice package. Each is wrapped with %w so
+// callers can test for a category while keeping the cause.
+var (
+	// ErrProviderFailed is returned when every provider in a chain fails.
+	ErrProviderFailed = errors.New("voice: all providers failed")
+	// ErrNotConfigured is returned when a provider has no endpoint/setting.
+	ErrNotConfigured = errors.New("voice: provider not configured")
+	// ErrNotImplemented is returned by an adaptation-point provider.
+	ErrNotImplemented = errors.New("voice: provider not implemented")
+	// ErrEmptyText is returned when TTS receives empty text.
+	ErrEmptyText = errors.New("voice: empty text")
+	// ErrClosed is returned when a service call arrives after Close.
+	ErrClosed = errors.New("voice: service closed")
+)