feat: M3 — 伏笔账本 + 节奏引擎 + 大纲（含并发记账 bugfix）

- 伏笔账本：纯函数状态机(OPEN/PARTIAL/CLOSED/OVERDUE) + ForeshadowLedger repo；验收后到期扫描(BackgroundTask 自建 session 置 OVERDUE)；登记/状态变更端点
- 节奏 + 三审齐：foreshadow-analyst + pace-checker 并入 LangGraph 并行审(REVIEW_SPECS)，collect 分列落 chapter_reviews(conflicts/foreshadow_sug/pace)，review SSE 加 foreshadow/pace 事件
- 大纲：outliner Agent 产 OutlineResult(含 foreshadow_windows)，POST /outline 逐章 upsert outline 表；GET /foreshadow?status= 看板
- 前端：伏笔四泳道看板(OVERDUE 琥珀) + 大纲编辑器(窗口徽标) + 节奏节拍图(▁▃▅) + 审稿页消费 foreshadow/pace SSE
- bugfix(T3.8)：并行三审共用请求 session 记账触发 'Session is already flushing' → foreshadow/pace 静默丢失；SqlAlchemyLedgerSink.record 改 add-only(靠端点/事务 commit)，加并发回归测试
- M3 E2E：真实 DB + mock 网关零 token 走通 埋设→进展→验收后扫描 OVERDUE→看板 + 大纲含窗口 + 三审齐 SSE/留痕；E2E 暴露并钉住上述 bug
- 门禁绿：mypy 111 / pytest 228(0 xfailed) / alembic 无漂移；前端 gen:api/lint/tsc/vitest 69/build

apps/web/components/foreshadow/ForeshadowBoard.tsx

@@ -0,0 +1,72 @@
+"use client";
+
+import { useMemo } from "react";
+
+import { AppShell } from "@/components/AppShell";
+import type { ForeshadowView, ProjectResponse } from "@/lib/api/types";
+import {
+  LANES,
+  groupByStatus,
+  type ForeshadowStatus,
+} from "@/lib/foreshadow/board";
+import { useForeshadow } from "@/lib/foreshadow/useForeshadow";
+import { KanbanColumn } from "./KanbanColumn";
+import { RegisterForm } from "./RegisterForm";
+
+interface ForeshadowBoardProps {
+  project: ProjectResponse;
+  initialItems: ForeshadowView[];
+}
+
+// 伏笔看板主体（UX §6.8）：四泳道 + 登记/状态变更交互。
+// RSC 取全量种入；Client 承载登记/转移（乐观 + 回滚 + Toast）。
+export function ForeshadowBoard({
+  project,
+  initialItems,
+}: ForeshadowBoardProps) {
+  const { items, busy, register, transition } = useForeshadow(initialItems);
+  const lanes = useMemo(() => groupByStatus(items), [items]);
+
+  const onTransition = (
+    code: string,
+    toStatus: ForeshadowStatus | null,
+    progressNote: string,
+  ): void => {
+    void transition(code, {
+      projectId: project.id,
+      toStatus,
+      progressEntry: progressNote ? { note: progressNote } : null,
+    });
+  };
+
+  return (
+    <AppShell
+      title={`〈${project.title}〉`}
+      subtitle="伏笔看板"
+      projectId={project.id}
+      activeNav="foreshadow"
+    >
+      <div className="flex h-[calc(100vh-4rem)] flex-col p-4">
+        <div className="mb-4 flex items-start justify-between gap-4">
+          <h1 className="font-serif text-lg text-ink">伏笔看板</h1>
+          <RegisterForm
+            projectId={project.id}
+            busy={busy}
+            onRegister={register}
+          />
+        </div>
+        <div className="grid min-h-0 flex-1 grid-cols-1 gap-3 md:grid-cols-2 xl:grid-cols-4">
+          {LANES.map((status) => (
+            <KanbanColumn
+              key={status}
+              status={status}
+              items={lanes[status]}
+              busy={busy}
+              onTransition={onTransition}
+            />
+          ))}
+        </div>
+      </div>
+    </AppShell>
+  );
+}