feat: 引入 Eino 框架，实现四阶段生成管线

基于 compose.Graph 编排 PromptBuilder → AssetGenerator → QualitySupervisor → FormatAdapter，
含质检重试分支（最多 3 次）和降级输出。推理层提供 mock 模式，可替换为真实 API。

backend/internal/service/pipeline_test.go

@@ -0,0 +1,129 @@
+package service
+
+import (
+	"context"
+	"testing"
+)
+
+func TestPipeline_HappyPath(t *testing.T) {
+	// 质检一次通过
+	QualityChecker = func(_ context.Context, _ []GeneratedImage, _ map[string]string) (bool, string, error) {
+		return true, "", nil
+	}
+	defer func() { QualityChecker = defaultCheckQuality }()
+
+	output, err := RunPipeline(context.Background(), PipelineInput{
+		Prompt:    "一个拿剑的小人",
+		AssetType: "sprite",
+		ProjectStyle: map[string]string{
+			"artStyle": "pixel",
+			"palette":  "warm",
+		},
+		Params: AssetParams{
+			Resolution: 64,
+			Format:     "spritesheet",
+		},
+	})
+	if err != nil {
+		t.Fatalf("RunPipeline failed: %v", err)
+	}
+
+	if len(output.Assets) == 0 {
+		t.Fatal("expected non-empty assets")
+	}
+	if output.Metadata.FrameWidth != 64 {
+		t.Errorf("expected FrameWidth=64, got %d", output.Metadata.FrameWidth)
+	}
+	if output.Metadata.FrameHeight != 64 {
+		t.Errorf("expected FrameHeight=64, got %d", output.Metadata.FrameHeight)
+	}
+}
+
+func TestPipeline_RetryThenPass(t *testing.T) {
+	// 质检前 2 次 fail，第 3 次 pass
+	QualityChecker = NewCountedQualityChecker(3)
+	defer func() { QualityChecker = defaultCheckQuality }()
+
+	output, err := RunPipeline(context.Background(), PipelineInput{
+		Prompt:    "一把火焰剑",
+		AssetType: "sprite",
+		Params: AssetParams{
+			Resolution: 32,
+			Frames: FrameParams{
+				Directions:       4,
+				FramesPerDirection: 2,
+			},
+			Format: "spritesheet",
+		},
+	})
+	if err != nil {
+		t.Fatalf("RunPipeline failed: %v", err)
+	}
+
+	// 4 directions × 2 frames = 8 张图
+	if len(output.Assets) != 8 {
+		t.Errorf("expected 8 assets, got %d", len(output.Assets))
+	}
+	if output.Metadata.FrameCount != 8 {
+		t.Errorf("expected FrameCount=8, got %d", output.Metadata.FrameCount)
+	}
+	if output.Metadata.Directions != 4 {
+		t.Errorf("expected Directions=4, got %d", output.Metadata.Directions)
+	}
+}
+
+func TestPipeline_MaxRetryDegrade(t *testing.T) {
+	// 质检始终 fail，超过 3 次后降级输出
+	QualityChecker = AlwaysFailQualityChecker()
+	defer func() { QualityChecker = defaultCheckQuality }()
+
+	output, err := RunPipeline(context.Background(), PipelineInput{
+		Prompt:    "一只飞龙",
+		AssetType: "sprite",
+		Params: AssetParams{
+			Resolution: 48,
+		},
+	})
+	if err != nil {
+		t.Fatalf("RunPipeline failed: %v", err)
+	}
+
+	// 降级也应该有输出
+	if len(output.Assets) == 0 {
+		t.Fatal("expected non-empty assets even on degrade")
+	}
+	if output.Metadata.FrameWidth != 48 {
+		t.Errorf("expected FrameWidth=48, got %d", output.Metadata.FrameWidth)
+	}
+}
+
+func TestPipeline_StyleMerge(t *testing.T) {
+	// 验证风格合并：task 覆盖 project
+	QualityChecker = func(_ context.Context, _ []GeneratedImage, style map[string]string) (bool, string, error) {
+		// 验证合并结果
+		if style["artStyle"] != "realistic" {
+			t.Errorf("expected artStyle=realistic (task override), got %s", style["artStyle"])
+		}
+		if style["palette"] != "warm" {
+			t.Errorf("expected palette=warm (from project), got %s", style["palette"])
+		}
+		return true, "", nil
+	}
+	defer func() { QualityChecker = defaultCheckQuality }()
+
+	_, err := RunPipeline(context.Background(), PipelineInput{
+		Prompt:    "测试风格合并",
+		AssetType: "sprite",
+		ProjectStyle: map[string]string{
+			"artStyle": "pixel",
+			"palette":  "warm",
+		},
+		TaskStyle: map[string]string{
+			"artStyle": "realistic", // 覆盖 project
+		},
+		Params: AssetParams{Resolution: 64},
+	})
+	if err != nil {
+		t.Fatalf("RunPipeline failed: %v", err)
+	}
+}