feat(server): add layer-separation generator task

Introduce the `layer-separation` generator (edit-elements node action)
that splits a source image into a clean background layer, foreground
image artifacts with bounding boxes, and editable text render data.

- New layer_separation.go builds the separated result and artifacts
- SubmitGeneratorTask routes layer-separation and move-object via a
  switch; standalone task completion + polling support
- Extend artifact metadata with BBox and GeneratorName across the
  domain, API types, mapper, .api schema, and API.md
- edit-elements gets a transparent manual-frame placeholder holding the
  separated foreground images and text layers beside the original

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

server/internal/application/design_service.go

@@ -892,12 +892,14 @@ func (s *DesignService) RunNodeActionAsync(ctx context.Context, projectID string
 	now := s.now()
 	threadID := newID()
 	target := project.Nodes[nodeIndex]
-	if req.Action == "remove-background" || req.Action == "vectorize" || req.Action == "move-object" {
+	if req.Action == "remove-background" || req.Action == "vectorize" || req.Action == "move-object" || req.Action == "edit-elements" {
 		resultNode := newBackgroundRemovalPlaceholder(target)
 		if req.Action == "vectorize" {
 			resultNode = newVectorizePlaceholder(target)
 		} else if req.Action == "move-object" {
 			resultNode = newMoveObjectPlaceholder(target)
+		} else if req.Action == "edit-elements" {
+			resultNode = newLayerSeparationPlaceholder(target)
 		}
 		if req.Options == nil {
 			req.Options = make(map[string]string)
@@ -1845,7 +1847,10 @@ func (s *DesignService) completeNodeActionGeneration(ctx context.Context, projec
 	if req.Action == "vectorize" {
 		return s.completeVectorizeGeneration(ctx, projectID, threadID, target, req)
 	}
-	if req.Action == "edit-elements" || req.Action == "edit-text" {
+	if req.Action == "edit-elements" {
+		return s.completeLayerSeparationGeneration(ctx, projectID, threadID, target, req)
+	}
+	if req.Action == "edit-text" {
 		return s.completeEditableLayerGeneration(ctx, projectID, threadID, target, req)
 	}
 	editToolContent := "正在根据选中图片执行模型编辑，完成后会替换画布中的图片节点。"
@@ -4709,8 +4714,30 @@ func newMoveObjectPlaceholder(target design.Node) design.Node {
 	return result
 }
 
+func newLayerSeparationPlaceholder(target design.Node) design.Node {
+	result := target
+	result.ID = newID()
+	result.Type = design.NodeTypeFrame
+	result.Title = nodeActionWorkingTitle("edit-elements")
+	result.X = target.X + target.Width + 32
+	result.Y = target.Y
+	result.Content = target.Content
+	result.Tone = "frame"
+	result.Status = "generating"
+	result.ParentID = ""
+	result.LayerRole = "manual-frame"
+	result.FillColor = "transparent"
+	result.StrokeColor = "#C7C7C7"
+	result.StrokeWidth = 1
+	result.StrokeStyle = "solid"
+	result.MockupSurface = design.MockupSurface{}
+	result.MockupModel = design.MockupModel{}
+	result.MockupSourceContent = ""
+	return result
+}
+
 func nodeActionResultNodeID(target design.Node, req design.NodeActionRequest) string {
-	if (req.Action == "remove-background" || req.Action == "vectorize" || req.Action == "move-object") && len(req.Options) > 0 {
+	if (req.Action == "remove-background" || req.Action == "vectorize" || req.Action == "move-object" || req.Action == "edit-elements") && len(req.Options) > 0 {
 		if id := strings.TrimSpace(req.Options[nodeActionResultNodeIDOption]); id != "" {
 			return id
 		}