feat(server): place generated images beside the reference anchor node

Read a "reference anchor node:" agent directive and lay newly generated
image nodes immediately to the right of that node, inheriting its row and
falling back to its size, for planned, follow-up, and incremental flows.
Generalize the directive parser shared with the image-generator target.

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

server/internal/application/generation_failure_test.go

@@ -623,6 +623,62 @@ func TestAgentChatImageModeBypassesPlannerAndHidesSettings(t *testing.T) {
 	}
 }
 
+func TestAgentChatPlacesReferenceAnchoredPlaceholdersBesideLastCapsule(t *testing.T) {
+	ctx := context.Background()
+	repo := memory.NewProjectRepository()
+	service := NewDesignService(repo, nil, nil, nil, nil, fakeCreativeChatAgent{
+		decision: design.AgentDecision{Intent: "image"},
+	})
+	service.SetJobQueue(&fakeJobQueue{})
+	project := design.Project{
+		ID:     "project-reference-anchor",
+		Title:  "Reference anchor",
+		Status: design.StatusReady,
+		Nodes: []design.Node{
+			{ID: "reference-1", Type: design.NodeTypeImage, X: 40, Y: 80, Width: 400, Height: 300, Content: "https://example.com/reference-1.webp", Status: "success"},
+			{ID: "reference-2", Type: design.NodeTypeImage, X: 620, Y: 160, Width: 520, Height: 360, Content: "https://example.com/reference-2.webp", Status: "success"},
+		},
+		UpdatedAt: time.Now(),
+	}
+	if err := repo.Save(ctx, project); err != nil {
+		t.Fatal(err)
+	}
+
+	thread, err := service.AgentChat(ctx, project.ID, design.AgentChatRequest{
+		Mode:       "image",
+		ImageSize:  "1024x1024",
+		ImageCount: 2,
+		Messages: []design.AgentChatMessage{
+			{
+				Role: "user",
+				Contents: []design.AgentContent{
+					{Type: "image", ImageURL: project.Nodes[0].Content, ImageName: "first"},
+					{Type: "image", ImageURL: project.Nodes[1].Content, ImageName: "last"},
+					{Type: "text", Text: "把最后一张参考图转换成立面效果", TextSource: "input"},
+					{Type: "text", Text: "Reference anchor node: reference-2", TextSource: "settings"},
+				},
+			},
+		},
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(thread.Project.Nodes) != 4 {
+		t.Fatalf("expected two references and two placeholders, got %#v", thread.Project.Nodes)
+	}
+	if thread.Project.Nodes[1].Content != project.Nodes[1].Content || thread.Project.Nodes[1].X != project.Nodes[1].X {
+		t.Fatalf("expected last reference to remain unchanged, got %#v", thread.Project.Nodes[1])
+	}
+	first := thread.Project.Nodes[2]
+	second := thread.Project.Nodes[3]
+	if first.X <= project.Nodes[1].X+project.Nodes[1].Width || first.Y != project.Nodes[1].Y {
+		t.Fatalf("expected first placeholder beside last capsule, got %#v", first)
+	}
+	if second.X <= first.X+first.Width || second.Y != project.Nodes[1].Y {
+		t.Fatalf("expected second placeholder after first placeholder, got %#v", second)
+	}
+}
+
 func TestAgentChatCreatesPlaceholdersFromModelPlannedImageTasks(t *testing.T) {
 	ctx := context.Background()
 	repo := memory.NewProjectRepository()