refactor(embedding): drop init(), build default registry eagerly

Replace the import-time init() that mutated the DefaultRegistry global with a constructor that returns a ready registry, removing mutate-after-init global state. Exported API unchanged.

internal/embedding/service.go

@@ -562,17 +562,6 @@ func (m *bgeModel) Close() error {
 	return nil
 }
 
-// Register the BGE model with the default registry at init time
-func init() {
-	RegisterModel(ModelMetadata{
-		Name:        BGEModelName,
-		Version:     BGEModelVersion,
-		Dimensions:  EmbeddingDim,
-		Description: "High-quality semantic search model",
-		Default:     true,
-	}, newBGEModel)
-}
-
 // Service provides thread-safe text embedding generation with model abstraction.
 type Service struct {
 	model EmbeddingModel