feat(ux): T4-b 前端 — 本章指令框 + 风格快捷预设传入 draft 生成

新 lib/workbench/directive.ts (STYLE_PRESETS + composeDirective 纯函数+测)；useDraftStream.start 加 directive 参数（非空时 POST JSON body）；Workbench 加「本章指令」textarea + 风格预设 chips，写本章时 composeDirective 传入。TDD: directive 测。前端门禁绿。docs(progress): 记 Tier4 完成。

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

@@ -0,0 +1,26 @@
+import { describe, expect, it } from "vitest";
+
+import { composeDirective, STYLE_PRESETS } from "./directive";
+
+describe("composeDirective", () => {
+  it("returns empty string when nothing selected", () => {
+    expect(composeDirective("", [])).toBe("");
+    expect(composeDirective("   ", [])).toBe("");
+  });
+
+  it("uses free text only when no presets", () => {
+    expect(composeDirective("多写战斗", [])).toBe("多写战斗");
+  });
+
+  it("combines presets (in canonical order) then free text", () => {
+    const result = composeDirective("多写战斗", ["fast-pace", "less-ai"]);
+    const lessAi = STYLE_PRESETS.find((p) => p.id === "less-ai")!.text;
+    const fastPace = STYLE_PRESETS.find((p) => p.id === "fast-pace")!.text;
+    expect(result).toBe(`${lessAi}；${fastPace}；多写战斗`);
+  });
+
+  it("ignores unknown preset ids", () => {
+    const lessAi = STYLE_PRESETS.find((p) => p.id === "less-ai")!.text;
+    expect(composeDirective("", ["nope", "less-ai"])).toBe(lessAi);
+  });
+});