feat: 提示词优化支持多布局模式识别与纯白背景统一

- buildMetaPrompt 按素材类型给出双模式指令（单素材/网格瓦片集），LLM 根据用户意图选择
- 新增 isSheetRequest 自动识别精灵表/瓦片集/tileset 关键字
- 模板回退链路全面支持 isSheet 双模式，默认单素材模式
- 所有素材类型统一纯白色背景(#FFFFFF)，由后期 format 节点清洗去背
- 补充 sprite/background/ui/animation 四类素材的完整生成场景覆盖

backend/internal/service/prompt_agent_test.go

@@ -31,14 +31,30 @@ func TestRunPromptAgent_Fallback(t *testing.T) {
 
 func TestRunPromptAgent_Sprite(t *testing.T) {
 	output, err := RunPromptAgent(context.Background(), PromptAgentInput{
-		Tags:      []string{"像素", "中世纪", "战士"},
+		Tags:      []string{"像素", "中世纪", "战士", "精灵表"},
 		AssetType: "sprite",
 	})
 	if err != nil {
 		t.Fatalf("RunPromptAgent failed: %v", err)
 	}
 	if !strings.Contains(output.Prompt, "spritesheet") {
-		t.Errorf("sprite output should mention spritesheet: %s", output.Prompt)
+		t.Errorf("sprite sheet output should mention spritesheet: %s", output.Prompt)
+	}
+}
+
+func TestRunPromptAgent_SpriteSingle(t *testing.T) {
+	output, err := RunPromptAgent(context.Background(), PromptAgentInput{
+		Tags:      []string{"像素", "中世纪", "战士"},
+		AssetType: "sprite",
+	})
+	if err != nil {
+		t.Fatalf("RunPromptAgent failed: %v", err)
+	}
+	if !strings.Contains(output.Prompt, "纯白色背景") {
+		t.Errorf("single sprite output should mention 纯白色背景: %s", output.Prompt)
+	}
+	if strings.Contains(output.Prompt, "spritesheet") {
+		t.Errorf("single sprite output should not mention spritesheet: %s", output.Prompt)
 	}
 }
 
@@ -251,8 +267,9 @@ func TestBuildMetaPrompt_NoUserNote(t *testing.T) {
 
 func TestParseTagsFromMeta(t *testing.T) {
 	meta := `用户选择标签: 像素、中世纪、战士
-素材类型: sprite`
-	tags, assetType := parseTagsFromMeta(meta)
+素材类型: sprite
+用户原始描述: 一个持剑角色`
+	tags, assetType, userPrompt := parseTagsFromMeta(meta)
 	if len(tags) != 3 {
 		t.Fatalf("expected 3 tags, got %d: %v", len(tags), tags)
 	}
@@ -262,26 +279,29 @@ func TestParseTagsFromMeta(t *testing.T) {
 	if assetType != "sprite" {
 		t.Errorf("expected assetType=sprite, got %s", assetType)
 	}
+	if userPrompt != "一个持剑角色" {
+		t.Errorf("expected userPrompt='一个持剑角色', got %s", userPrompt)
+	}
 }
 
 func TestParseTagsFromMeta_SingleTag(t *testing.T) {
 	meta := `用户选择标签: 赛博朋克
 素材类型: background`
-	tags, _ := parseTagsFromMeta(meta)
+	tags, _, _ := parseTagsFromMeta(meta)
 	if len(tags) != 1 || tags[0] != "赛博朋克" {
 		t.Errorf("expected [赛博朋克], got %v", tags)
 	}
 }
 
 func TestParseTagsFromMeta_Empty(t *testing.T) {
-	tags, assetType := parseTagsFromMeta("no tags here")
+	tags, assetType, _ := parseTagsFromMeta("no tags here")
 	if len(tags) != 0 || assetType != "" {
 		t.Errorf("expected empty, got tags=%v assetType=%s", tags, assetType)
 	}
 }
 
 func TestGenerateStructuredPrompt(t *testing.T) {
-	prompt := generateStructuredPrompt([]string{"像素", "战士"}, "sprite")
+	prompt := generateStructuredPrompt([]string{"像素", "战士", "精灵表"}, "sprite", true)
 	if !strings.HasPrefix(prompt, "【主题】") {
 		t.Error("prompt should start with 【主题】")
 	}
@@ -293,21 +313,36 @@ func TestGenerateStructuredPrompt(t *testing.T) {
 	}
 }
 
+func TestGenerateStructuredPrompt_Single(t *testing.T) {
+	prompt := generateStructuredPrompt([]string{"像素", "战士"}, "sprite", false)
+	if !strings.Contains(prompt, "独立PNG") {
+		t.Errorf("single sprite should contain 独立PNG: %s", prompt)
+	}
+	if strings.Contains(prompt, "spritesheet") {
+		t.Errorf("single sprite should not contain spritesheet: %s", prompt)
+	}
+}
+
 func TestBuildSubject(t *testing.T) {
 	tags := []string{"像素", "战士"}
 	tests := []struct {
 		assetType, want string
+		isSheet         bool
 	}{
-		{"sprite", "精灵图"},
-		{"background", "场景背景"},
-		{"ui", "UI元素"},
-		{"animation", "动画帧序列"},
-		{"unknown", "游戏素材"},
+		{"sprite", "精灵图", false},
+		{"sprite", "精灵表", true},
+		{"background", "场景背景", false},
+		{"background", "瓦片集", true},
+		{"ui", "UI元素", false},
+		{"ui", "瓦片集", true},
+		{"animation", "动画帧序列", false},
+		{"animation", "精灵表", true},
+		{"unknown", "游戏素材", false},
 	}
 	for _, tt := range tests {
-		result := buildSubject(tags, tt.assetType)
+		result := buildSubject(tags, tt.assetType, tt.isSheet)
 		if !strings.Contains(result, tt.want) {
-			t.Errorf("buildSubject(%q) = %s, want containing %q", tt.assetType, result, tt.want)
+			t.Errorf("buildSubject(%q, isSheet=%v) = %s, want containing %q", tt.assetType, tt.isSheet, result, tt.want)
 		}
 	}
 }
@@ -327,17 +362,48 @@ func TestBuildStyle(t *testing.T) {
 func TestBuildTechNotes(t *testing.T) {
 	tests := []struct {
 		assetType, want string
+		isSheet         bool
 	}{
-		{"sprite", "spritesheet"},
-		{"background", "1920x1080"},
-		{"ui", "九宫格"},
-		{"animation", "4方向x4帧"},
-		{"unknown", "PNG"},
+		// 默认单人模式
+		{"sprite", "纯白色背景", false},
+		{"background", "1920x1080", false},
+		{"ui", "九宫格", false},
+		{"animation", "4方向x4帧", false},
+		{"unknown", "PNG", false},
+		// 瓦片集/精灵表模式
+		{"sprite", "spritesheet", true},
+		{"background", "tileset", true},
+		{"ui", "瓦片集", true},
+		{"animation", "spritesheet", true},
 	}
 	for _, tt := range tests {
-		result := buildTechNotes(tt.assetType)
+		result := buildTechNotes(tt.assetType, tt.isSheet)
 		if !strings.Contains(result, tt.want) {
-			t.Errorf("buildTechNotes(%q) = %s, want containing %q", tt.assetType, result, tt.want)
+			t.Errorf("buildTechNotes(%q, isSheet=%v) = %s, want containing %q", tt.assetType, tt.isSheet, result, tt.want)
+		}
+	}
+}
+
+func TestIsSheetRequest(t *testing.T) {
+	tests := []struct {
+		tags   []string
+		prompt string
+		want   bool
+	}{
+		{[]string{"像素", "精灵表"}, "", true},
+		{[]string{"像素", "spritesheet"}, "", true},
+		{[]string{"地形", "瓦片集"}, "", true},
+		{[]string{"UI", "tileset"}, "", true},
+		{[]string{"场景", "tilemap"}, "", true},
+		{[]string{"像素", "战士"}, "", false},
+		{[]string{"像素"}, "生成一个精灵表", true},
+		{[]string{"森林"}, "场景瓦片集", true},
+		{nil, "", false},
+	}
+	for _, tt := range tests {
+		got := isSheetRequest(tt.tags, tt.prompt)
+		if got != tt.want {
+			t.Errorf("isSheetRequest(tags=%v, prompt=%q) = %v, want %v", tt.tags, tt.prompt, got, tt.want)
 		}
 	}
 }