feat(frontend): AI写作三槽——文风 / 剧情需求 / 自定义要求，写章前按需组合

Phase 2（写作工作台重构）：把写章指令升级为三槽——文风预设 + 新增剧情需求预设
（打脸反转/章末钩子/高潮/转折/扮猪吃虎，源自 plan §4.2 分类法，tianyayu6/oh-story MIT）
+ 自定义自由文本。三者经 composeDirective 合并为单条 directive→volatile（MVP 纯前端、
零契约、不动 write_craft.md 字节，金标准零回归）。DirectivePanel 拆成文风/剧情两组 chips。

apps/web/lib/workbench/directive.test.ts

@@ -1,6 +1,6 @@
 import { describe, expect, it } from "vitest";
 
-import { composeDirective, STYLE_PRESETS } from "./directive";
+import { composeDirective, PLOT_PRESETS, STYLE_PRESETS } from "./directive";
 
 describe("composeDirective", () => {
   it("returns empty string when nothing selected", () => {
@@ -23,4 +23,16 @@ describe("composeDirective", () => {
     const lessAi = STYLE_PRESETS.find((p) => p.id === "less-ai")!.text;
     expect(composeDirective("", ["nope", "less-ai"])).toBe(lessAi);
   });
+
+  it("resolves 剧情需求 presets alongside 文风", () => {
+    const plot = PLOT_PRESETS[0]!;
+    expect(composeDirective("", [plot.id])).toBe(plot.text);
+  });
+
+  it("orders 文风 presets before 剧情 presets, then free text", () => {
+    const style = STYLE_PRESETS[0]!;
+    const plot = PLOT_PRESETS[0]!;
+    const result = composeDirective("再收紧节奏", [plot.id, style.id]);
+    expect(result).toBe(`${style.text}；${plot.text}；再收紧节奏`);
+  });
 });