feat(ux): B0 注入透明读端点 + F1 写作页右栏真面板

兑现「看到的=写章用的」信任牌（不变量 #6），替换写作页右栏过时假占位
（「M1 暂未接 / M2 开放」）。

后端：
- GET /projects/{id}/chapters/{no}/injection → InjectionResponse（selected
  实体 + 入选理由 + recent_n）；调既有 assemble() 回放确定性 SelectionTrace，
  无 LLM / 无 commit / 无 DDL；项目不存在→404，无大纲→selected:[]。
- 新 schemas/injection.py；tests/test_injection.py(3 测)。

前端：
- gen:api 纳入端点；纯逻辑 lib/workbench/injection.ts（理由/类型→中文徽标
  + 4 vitest）+ useInjection 读 hook。
- ChapterAssistant 改 client 组件：列出选中实体 + 理由徽标 + 空/载/错三态，
  四审段指向审稿页；Workbench 传 projectId/chapterNo。

门禁：后端 ruff/format/mypy 159/alembic 无漂移/pytest 454；
前端 lint/typecheck/vitest 167/build。

B0 可控版（PUT override pin/排除/recent_n + selection 加参 + draft 同读
override + 持久化）+ F1 可控控件待后续。

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

@@ -0,0 +1,27 @@
+import { describe, expect, it } from "vitest";
+
+import { kindLabel, reasonLabel } from "./injection";
+
+describe("reasonLabel", () => {
+  it("maps each known selection reason to a Chinese badge", () => {
+    expect(reasonLabel("explicit_beat")).toBe("本章点名");
+    expect(reasonLabel("main_character")).toBe("主角常驻");
+    expect(reasonLabel("recent_digest")).toBe("近章出现");
+    expect(reasonLabel("foreshadow_window")).toBe("伏笔窗口");
+  });
+
+  it("falls back to the raw code for an unknown reason", () => {
+    expect(reasonLabel("future_reason")).toBe("future_reason");
+  });
+});
+
+describe("kindLabel", () => {
+  it("maps entity kinds to Chinese", () => {
+    expect(kindLabel("character")).toBe("角色");
+    expect(kindLabel("world_entity")).toBe("设定");
+  });
+
+  it("falls back to the raw kind when unknown", () => {
+    expect(kindLabel("mystery")).toBe("mystery");
+  });
+});

apps/web/lib/workbench/injection.ts

@@ -0,0 +1,30 @@
+// 本章注入透明（B0/F1）纯逻辑：入选理由 / 实体类型 → 中文徽标。
+// 纯函数，便于 node 环境单测；渲染层只管样式。
+
+import type { components } from "@/lib/api/schema";
+
+export type InjectionResponse = components["schemas"]["InjectionResponse"];
+export type InjectionEntity = components["schemas"]["InjectionEntity"];
+
+// SelectionReason → 作者可读徽标（对齐 ARCH §3.4 的四来源）。
+export const REASON_LABELS: Record<string, string> = {
+  explicit_beat: "本章点名",
+  main_character: "主角常驻",
+  recent_digest: "近章出现",
+  foreshadow_window: "伏笔窗口",
+};
+
+// EntityKind → 中文。
+export const KIND_LABELS: Record<string, string> = {
+  character: "角色",
+  world_entity: "设定",
+};
+
+// 未知 code 时回退原值（不吞，便于发现新理由）。
+export function reasonLabel(reason: string): string {
+  return REASON_LABELS[reason] ?? reason;
+}
+
+export function kindLabel(kind: string): string {
+  return KIND_LABELS[kind] ?? kind;
+}

apps/web/lib/workbench/useInjection.ts

@@ -0,0 +1,50 @@
+"use client";
+
+import { useEffect, useState } from "react";
+
+import { api } from "@/lib/api/client";
+import type { InjectionResponse } from "./injection";
+
+export interface UseInjection {
+  data: InjectionResponse | null;
+  loading: boolean;
+  error: string | null;
+}
+
+// 本章注入透明（B0 读端点）：挂载时拉一次确定性 SelectionTrace。
+// 只读、无 LLM、无 commit——展示「选了谁/为什么」。失败给可读文案不抛。
+export function useInjection(
+  projectId: string,
+  chapterNo: number,
+): UseInjection {
+  const [data, setData] = useState<InjectionResponse | null>(null);
+  const [loading, setLoading] = useState(true);
+  const [error, setError] = useState<string | null>(null);
+
+  useEffect(() => {
+    let cancelled = false;
+    setLoading(true);
+    setError(null);
+
+    void (async () => {
+      const { data: body, error: err } = await api.GET(
+        "/projects/{project_id}/chapters/{chapter_no}/injection",
+        { params: { path: { project_id: projectId, chapter_no: chapterNo } } },
+      );
+      if (cancelled) return;
+      if (err || !body) {
+        setError("注入信息暂不可用");
+        setData(null);
+      } else {
+        setData(body);
+      }
+      setLoading(false);
+    })();
+
+    return () => {
+      cancelled = true;
+    };
+  }, [projectId, chapterNo]);
+
+  return { data, loading, error };
+}