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/stream/useDraftStream.ts

@@ -41,7 +41,11 @@ function reducer(state: StreamState, action: Action): StreamState {
 export interface UseDraftStream {
   state: StreamState;
   isStreaming: boolean;
-  start: (projectId: string, chapterNo: number) => Promise<void>;
+  start: (
+    projectId: string,
+    chapterNo: number,
+    directive?: string,
+  ) => Promise<void>;
   stop: () => void;
   reset: (text: string) => void;
 }
@@ -62,55 +66,72 @@ export function useDraftStream(): UseDraftStream {
     dispatch({ type: "reset", text });
   }, []);
 
-  const start = useCallback(async (projectId: string, chapterNo: number) => {
-    const controller = new AbortController();
-    controllerRef.current = controller;
-    dispatch({ type: "start" });
-    try {
-      const res = await fetch(
-        `${API_BASE_PUBLIC}/projects/${projectId}/chapters/${chapterNo}/draft`,
-        {
-          method: "POST",
-          headers: { Accept: "text/event-stream" },
-          signal: controller.signal,
-        },
-      );
-      if (!res.ok || !res.body) {
-        // 流前错误（如无凭据 → 503 LLM_UNAVAILABLE，JSON 信封而非帧）。
-        let code = "STREAM_FAILED";
-        let message = `写章请求失败（${res.status}）`;
-        try {
-          const body = (await res.json()) as {
-            error?: { code?: string; message?: string };
-          };
-          if (body.error?.code) code = body.error.code;
-          if (body.error?.message) message = body.error.message;
-        } catch {
-          // 非 JSON 信封，沿用默认文案。
+  const start = useCallback(
+    async (projectId: string, chapterNo: number, directive?: string) => {
+      const controller = new AbortController();
+      controllerRef.current = controller;
+      dispatch({ type: "start" });
+      try {
+        // 仅当本章指令非空才发 JSON body（无指令退回裸 POST，与旧后端调用兼容）。
+        const trimmed = directive?.trim();
+        const init: RequestInit =
+          trimmed && trimmed.length > 0
+            ? {
+                method: "POST",
+                headers: {
+                  Accept: "text/event-stream",
+                  "Content-Type": "application/json",
+                },
+                body: JSON.stringify({ directive: trimmed }),
+                signal: controller.signal,
+              }
+            : {
+                method: "POST",
+                headers: { Accept: "text/event-stream" },
+                signal: controller.signal,
+              };
+        const res = await fetch(
+          `${API_BASE_PUBLIC}/projects/${projectId}/chapters/${chapterNo}/draft`,
+          init,
+        );
+        if (!res.ok || !res.body) {
+          // 流前错误（如无凭据 → 503 LLM_UNAVAILABLE，JSON 信封而非帧）。
+          let code = "STREAM_FAILED";
+          let message = `写章请求失败（${res.status}）`;
+          try {
+            const body = (await res.json()) as {
+              error?: { code?: string; message?: string };
+            };
+            if (body.error?.code) code = body.error.code;
+            if (body.error?.message) message = body.error.message;
+          } catch {
+            // 非 JSON 信封，沿用默认文案。
+          }
+          dispatch({ type: "fail", code, message });
+          return;
         }
-        dispatch({ type: "fail", code, message });
-        return;
+        const reader = res.body.getReader();
+        const decoder = new TextDecoder();
+        const buffer = new SseFrameBuffer();
+        for (;;) {
+          const { value, done } = await reader.read();
+          if (done) break;
+          const events = buffer.push(decoder.decode(value, { stream: true }));
+          if (events.length > 0) dispatch({ type: "events", events });
+        }
+      } catch (err: unknown) {
+        if (err instanceof DOMException && err.name === "AbortError") {
+          // 用户主动停止：state 已置 aborted。
+          return;
+        }
+        const message = err instanceof Error ? err.message : "未知网络错误";
+        dispatch({ type: "fail", code: "NETWORK", message });
+      } finally {
+        controllerRef.current = null;
       }
-      const reader = res.body.getReader();
-      const decoder = new TextDecoder();
-      const buffer = new SseFrameBuffer();
-      for (;;) {
-        const { value, done } = await reader.read();
-        if (done) break;
-        const events = buffer.push(decoder.decode(value, { stream: true }));
-        if (events.length > 0) dispatch({ type: "events", events });
-      }
-    } catch (err: unknown) {
-      if (err instanceof DOMException && err.name === "AbortError") {
-        // 用户主动停止：state 已置 aborted。
-        return;
-      }
-      const message = err instanceof Error ? err.message : "未知网络错误";
-      dispatch({ type: "fail", code: "NETWORK", message });
-    } finally {
-      controllerRef.current = null;
-    }
-  }, []);
+    },
+    [],
+  );
 
   return {
     state,

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

@@ -0,0 +1,41 @@
+// 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);
+}