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/lib/pace/beatmap.test.ts

@@ -0,0 +1,37 @@
+import { describe, expect, it } from "vitest";
+
+import { SPARK_CHARS, toBeatCells, toSparkline } from "./beatmap";
+
+describe("toBeatCells", () => {
+  it("maps min→▁ and max→▇ via linear normalization", () => {
+    const cells = toBeatCells([1, 7]);
+    expect(cells[0]?.level).toBe(0);
+    expect(cells[0]?.char).toBe("▁");
+    expect(cells[1]?.level).toBe(SPARK_CHARS.length - 1);
+    expect(cells[1]?.char).toBe("▇");
+  });
+
+  it("centers all cells when every value is equal", () => {
+    const cells = toBeatCells([5, 5, 5]);
+    expect(cells.every((c) => c.level === 3)).toBe(true);
+    expect(cells.every((c) => c.char === "▄")).toBe(true);
+  });
+
+  it("gives every cell a visible min height", () => {
+    const cells = toBeatCells([0, 10]);
+    expect(cells[0]?.heightPct).toBeGreaterThanOrEqual(12);
+    expect(cells[1]?.heightPct).toBe(100);
+  });
+
+  it("returns [] for empty or non-finite-only input", () => {
+    expect(toBeatCells([])).toEqual([]);
+    expect(toBeatCells([Number.NaN, Number.POSITIVE_INFINITY])).toEqual([]);
+  });
+});
+
+describe("toSparkline", () => {
+  it("joins chars into a sparkline string", () => {
+    expect(toSparkline([1, 4, 7])).toBe("▁▄▇");
+    expect(toSparkline([])).toBe("");
+  });
+});