feat(foreshadow): 写作台本章伏笔主动提醒（前端）

- 新增 chapterForeshadow 纯分类：按当前章分 可回收/待回收/已逾期（启用闲置的 isWindowApproaching）
- useChapterForeshadow hook（复用 GET /foreshadow，切章仅重分类不重拉）
- 右栏「本章参考」+ ContextDrawer 伏笔页新增「本章伏笔」实时清单，
  分色展示 + 跳看板链接 + 空态；标注『按已验收进度』

apps/web/lib/foreshadow/chapterForeshadow.test.ts

@@ -0,0 +1,104 @@
+import { describe, expect, it } from "vitest";
+
+import type { ForeshadowView } from "@/lib/api/types";
+import {
+  chapterForeshadowTotal,
+  classifyChapterForeshadow,
+  foreshadowStatusLabel,
+} from "./chapterForeshadow";
+
+const view = (over: Partial<ForeshadowView>): ForeshadowView => ({
+  code: "F-001",
+  title: "线索",
+  status: "OPEN",
+  ...over,
+});
+
+describe("classifyChapterForeshadow", () => {
+  it("puts items whose window covers the current chapter into 可回收", () => {
+    const groups = classifyChapterForeshadow(
+      [
+        view({ code: "F-A", status: "OPEN", expected_close_from: 40, expected_close_to: 60 }),
+        view({ code: "F-B", status: "PARTIAL", expected_close_to: 50 }),
+      ],
+      50,
+    );
+    expect(groups.recyclable.map((v) => v.code)).toEqual(["F-A", "F-B"]);
+    expect(groups.pending).toEqual([]);
+    expect(groups.overdue).toEqual([]);
+  });
+
+  it("puts OPEN/PARTIAL with a later or missing window into 待回收", () => {
+    const groups = classifyChapterForeshadow(
+      [
+        view({ code: "F-LATER", status: "OPEN", expected_close_from: 80, expected_close_to: 90 }),
+        view({ code: "F-NOWIN", status: "PARTIAL" }),
+      ],
+      50,
+    );
+    expect(groups.pending.map((v) => v.code)).toEqual(["F-LATER", "F-NOWIN"]);
+    expect(groups.recyclable).toEqual([]);
+  });
+
+  it("puts OVERDUE into 已逾期 even when a window would otherwise cover the chapter", () => {
+    const groups = classifyChapterForeshadow(
+      [view({ code: "F-OD", status: "OVERDUE", expected_close_from: 40, expected_close_to: 60 })],
+      50,
+    );
+    expect(groups.overdue.map((v) => v.code)).toEqual(["F-OD"]);
+    expect(groups.recyclable).toEqual([]);
+  });
+
+  it("drops CLOSED and unknown statuses from every group", () => {
+    const groups = classifyChapterForeshadow(
+      [
+        view({ code: "F-CLOSED", status: "CLOSED", expected_close_to: 60 }),
+        view({ code: "F-WEIRD", status: "WEIRD" }),
+      ],
+      50,
+    );
+    expect(chapterForeshadowTotal(groups)).toBe(0);
+  });
+
+  it("handles the window boundary: inclusive at from and to, excluded just outside", () => {
+    const win = (code: string): ForeshadowView =>
+      view({ code, status: "OPEN", expected_close_from: 40, expected_close_to: 60 });
+    expect(classifyChapterForeshadow([win("lo")], 40).recyclable).toHaveLength(1);
+    expect(classifyChapterForeshadow([win("hi")], 60).recyclable).toHaveLength(1);
+    expect(classifyChapterForeshadow([win("before")], 39).pending).toHaveLength(1);
+    expect(classifyChapterForeshadow([win("after")], 61).pending).toHaveLength(1);
+  });
+
+  it("returns empty groups for empty / undefined input without mutating a shared object", () => {
+    const a = classifyChapterForeshadow(undefined, 1);
+    const b = classifyChapterForeshadow([], 1);
+    expect(chapterForeshadowTotal(a)).toBe(0);
+    expect(chapterForeshadowTotal(b)).toBe(0);
+    // 不可变：两次调用返回相互独立的空组
+    a.pending.push(view({}));
+    expect(b.pending).toEqual([]);
+  });
+});
+
+describe("chapterForeshadowTotal", () => {
+  it("sums the three lanes", () => {
+    const groups = classifyChapterForeshadow(
+      [
+        view({ code: "r", status: "OPEN", expected_close_to: 50 }),
+        view({ code: "p", status: "OPEN" }),
+        view({ code: "o", status: "OVERDUE" }),
+      ],
+      50,
+    );
+    expect(chapterForeshadowTotal(groups)).toBe(3);
+  });
+});
+
+describe("foreshadowStatusLabel", () => {
+  it("maps known statuses to author-facing labels and falls back for unknown", () => {
+    expect(foreshadowStatusLabel("OPEN")).toBe("待推进");
+    expect(foreshadowStatusLabel("PARTIAL")).toBe("推进中");
+    expect(foreshadowStatusLabel("OVERDUE")).toBe("已逾期");
+    expect(foreshadowStatusLabel("WEIRD")).toBe("WEIRD");
+  });
+});