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);
+  });
+});

apps/web/lib/workbench/directive.ts

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+// T4-b · 本章指令组装（纯逻辑，UX §3）。
+// 作者自由文本 + 风格快捷预设 → 单条 directive 串，传入 draft 生成（直通后端 volatile）。
+// 组件只渲染 chips/textarea；预设清单与组装规则集中在此，确定性、不可变。
+
+export interface StylePreset {
+  id: string;
+  label: string;
+  text: string;
+}
+
+// 风格快捷预设（点选拼入指令开头，引导本章文风）。
+export const STYLE_PRESETS: readonly StylePreset[] = [
+  { id: "less-ai", label: "减少AI味", text: "减少 AI 腔，去掉套话与排比堆砌" },
+  { id: "colloquial", label: "口语化", text: "用更口语、贴近人物的表达" },
+  { id: "fast-pace", label: "快节奏", text: "加快节奏，少铺垫多冲突推进" },
+];
+
+const PRESET_BY_ID: ReadonlyMap<string, StylePreset> = new Map(
+  STYLE_PRESETS.map((preset) => [preset.id, preset]),
+);
+
+const SEPARATOR = "；";
+
+// 组合选中预设文案（按 STYLE_PRESETS 原序）+ 自由文本为一条指令。空 → ""。
+export function composeDirective(
+  freeText: string,
+  presetIds: readonly string[],
+): string {
+  const selected = new Set(presetIds);
+  const presetTexts = STYLE_PRESETS.filter((preset) =>
+    selected.has(preset.id),
+  ).map((preset) => preset.text);
+  const trimmedFree = freeText.trim();
+  const parts = trimmedFree ? [...presetTexts, trimmedFree] : presetTexts;
+  return parts.join(SEPARATOR);
+}
+
+// 仅暴露给测试/组件的辅助：id 是否为已知预设。
+export function isKnownPreset(id: string): boolean {
+  return PRESET_BY_ID.has(id);
+}