fix(storyboard): 章节分镜任务补输出契约并穿透 text 包装解析镜头行

- 章节分镜任务 prompt 显式追加 {title, rows} JSON 输出契约：该链路不走服务端分镜模板（无 metadata.promptTemplateOperation），缺契约时模型按自然语言习惯返回 Markdown 表格，导致“分镜任务没有返回镜头行”
- 文本任务 result_json 外层统一为 {mode, reasoning, text} 包装，storyboardRowsFromTask 现穿透 text 内层解析分镜表，并兜底剥离代码块与首尾大括号截取；顶层直接落 {title, rows} 的结构保持兼容
- 分镜任务 input 显式携带 mode: text（后端仅对 video_ 前缀任务兜底）
- 新增 canvas-storyboard-rows-parse.test.ts 覆盖六种解析场景

web/src/pages/projects/detail/project-chapter-ai.ts

@@ -1,6 +1,6 @@
 import { runBackendCanvasGenerationTask } from "@/lib/canvas/canvas-project-generation";
 import { PromptTemplateOperation, promptTemplateTaskPlaceholder } from "@/lib/prompts";
-import { storyboardRowsFromTask } from "@/lib/canvas/canvas-project-domain";
+import { storyboardRowsFromTask, storyboardRowsOutputContract } from "@/lib/canvas/canvas-project-domain";
 import { parseChapterAssetBreakdown, type ChapterAssetBreakdown } from "@/lib/canvas/chapter-asset-breakdown";
 import { parseCharacterBreakdown } from "@/lib/canvas/canvas-character-reference";
 import { backendProviderConfig, parseBackendGenerationResult } from "@/services/api/generation-task";
@@ -90,14 +90,21 @@ export async function generateChapterStoryboard(input: ChapterStoryboardGenerati
         skills: input.skills,
         selectedSkillIds: input.selectedSkillIds,
     });
+    // 技能上下文只描述工作流，不会约束输出结构；必须显式带上输出契约，否则模型返回 Markdown 表格。
+    const prompt = [
+        skillExecution.prompt,
+        storyboardRowsOutputContract("每个镜头必须能独立用于生成首帧图片和镜头视频。"),
+    ].join("\n\n");
     const task = await createGenerationTask({
         projectId: input.projectId,
         type: "canvas_text",
         operation: "storyboard",
-        prompt: skillExecution.prompt,
+        prompt,
         model,
         ...(logicalModelIDForConfig(config) ? { logicalModelId: logicalModelIDForConfig(config) } : {}),
         input: {
+            // 后端只在 video_ 前缀的任务上兜底 mode，文本任务必须显式带上。
+            mode: "text",
             canvasAssets: input.assets,
             requirements: "输出可直接写入分镜制作并继续生成分镜图、动作预演和镜头视频的分镜表。",
             projectStyle: input.projectStyle,