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/vision/errors.go

@@ -0,0 +1,22 @@
+package vision
+
+import "errors"
+
+// Sentinel errors returned by the vision package. Each is wrapped with %w on
+// the way out so callers can test for a category while keeping the cause.
+var (
+	// ErrNoImage is returned when an image source string is empty.
+	ErrNoImage = errors.New("vision: empty image source")
+	// ErrNotConfigured is returned by adapters when no endpoint/model is set.
+	ErrNotConfigured = errors.New("vision: adapter not configured")
+	// ErrUnsupportedSource is returned when an image source cannot be handled.
+	ErrUnsupportedSource = errors.New("vision: unsupported image source")
+	// ErrFetch is returned when a URL image cannot be downloaded.
+	ErrFetch = errors.New("vision: cannot fetch image")
+	// ErrUpstream is returned when the vision/OCR provider request fails.
+	ErrUpstream = errors.New("vision: upstream request failed")
+	// ErrBadImage is returned when an image cannot be read or decoded.
+	ErrBadImage = errors.New("vision: cannot read image")
+	// ErrBadArgs is returned by a tool when its arguments are malformed.
+	ErrBadArgs = errors.New("vision: bad tool arguments")
+)