接入边缘浏览器语音合成并做本地缓存，听书段落可直接朗读。

依赖随锁文件一并落下，避免环境缺包。

Co-authored-by: Cursor <cursoragent@cursor.com>

tests/tts-cache.test.ts

@@ -0,0 +1,37 @@
+import { describe, expect, it } from "vitest";
+import { ttsCacheKey } from "../src/tts/cache.js";
+import { DEFAULT_TTS_PROVIDER, resolveTtsProvider } from "../src/tts/index.js";
+
+describe("tts cache / provider", () => {
+  it("hashes stable keys", () => {
+    const a = ttsCacheKey({
+      provider: "edge",
+      voice: "zh-CN-XiaoxiaoNeural",
+      text: "你好",
+      extra: "s1",
+    });
+    const b = ttsCacheKey({
+      provider: "edge",
+      voice: "zh-CN-XiaoxiaoNeural",
+      text: "你好",
+      extra: "s1",
+    });
+    const c = ttsCacheKey({
+      provider: "edge",
+      voice: "zh-CN-XiaoxiaoNeural",
+      text: "你好啊",
+      extra: "s1",
+    });
+    expect(a).toBe(b);
+    expect(a).not.toBe(c);
+    expect(a.length).toBe(40);
+  });
+
+  it("defaults provider to edge", () => {
+    expect(DEFAULT_TTS_PROVIDER).toBe("edge");
+    const prev = process.env.WRITING_AGENT_TTS_PROVIDER;
+    delete process.env.WRITING_AGENT_TTS_PROVIDER;
+    expect(resolveTtsProvider()).toBe("edge");
+    process.env.WRITING_AGENT_TTS_PROVIDER = prev;
+  });
+});