音频改走无扩展名的 /media 路由：IDM / 迅雷 这类下载管理器不再弹「下载文件信息」

玩家那边遇到的是：游戏放着 BGM，突然弹出下载管理器的「下载文件信息」框，有时还会把整条
音轨抢去下载。原因是这些工具会嗅探以媒体扩展名结尾的 XHR/fetch。改成客户端统一请求不带
扩展名的 `/media/<原路径去掉 .mp3>`，服务端再解析回 `public/assets/audio/…`。

- `public/js/media.js`：`mediaUrl()` 把 `/assets/audio/**.mp3` 换算成 `/media/**`，只处理同源、
  且扩展名在名单内的地址；跨域、目录、`..`、点开头、不认识的扩展名一律原样返回。
- `server/index.js`：新增 `/media/` 路由。按 `AUDIO_EXTS` 顺序找 `.mp3/.m4a/.aac/.ogg/.oga/.opus/.wav`，
  显式写了扩展名的以它为准；Range/ETag/Content-Type/缓存策略都复用 `serveFile`，所以和直接请求
  `/assets/audio/…` 行为一致（一天缓存、支持 Range）。
- 直接请求 `/assets/audio/…` 仍然可用 —— 这是给不支持 `/media/` 的纯静态托管留的退路，客户端
  在 `/media/` 404 时才回退到原地址。

安全：`/media/` 的路径只接受正常片段，`..`/`.` 直接 403，点开头或点结尾的片段 404，解析出的
绝对路径必须落在 `public/assets/audio` 之内，且只允许上面那几个音频扩展名 —— 不会变成任意文件读取。

测试：
- `test/media-url.test.js`：`mediaUrl()` 的换算与各种「别动它」的边界。
- `test/lobby.test.js`：真实服务器上的 `/media` 路由（正常解析、显式扩展名优先、Range、HEAD、
  目录与穿越请求被拒、客户端回退用到的 `/media/bgm.mp3` 仍 200）。
- `test/ui/audio.test.js`：音频管理器现在先请求无扩展名地址，404 才回退原地址；断言跟着
  `mediaUrl()` 换算后再比对（原先拿 manifest 原地址比对，等于断言一个客户端永不请求的 URL）。

public/js/audio.js

@@ -31,6 +31,7 @@
 // which follows the store).
 
 import { PHASE } from '../../shared/constants.js';
+import { mediaUrl } from './media.js';
 
 const MAX_VOICES = 8;
 const UNIT_COOLDOWN_MS = 160;
@@ -383,7 +384,10 @@ export class AudioManager {
     }
     const p = (async () => {
       try {
-        const res = await fetch(url);
+        // Extension-less URL first so download managers leave the BGM alone; a host without /media/ still works.
+        const media = mediaUrl(url);
+        let res = await fetch(media);
+        if (!res.ok && media !== url) res = await fetch(url);
         if (!res.ok) throw new Error(`HTTP ${res.status}`);
         const ab = await res.arrayBuffer();
         return await new Promise((resolve) => {

public/js/media.js

@@ -0,0 +1,60 @@
+// Audio URLs for download managers (IDM / 迅雷 / FDM …) — and why they have no extension.
+//
+// The game plays audio with fetch() + Web Audio (`audio.js`): it never uses <audio src>, <a download> or a
+// navigation. Download managers do not care — their browser integration hooks XHR/fetch whose URL ends in a
+// media extension and (usually above ~1 MB) pops up "下载文件信息" for every BGM track. That dialog has nothing
+// to do with the game, but players see it and blame the game.
+//
+// So audio is fetched through a same-origin, extension-less path instead:
+//
+//     /assets/audio/bgm/act1.mp3   →   /media/bgm/act1
+//
+// `server/index.js` resolves /media/… back to the real file under public/assets/audio and still answers with
+// `Content-Type: audio/mpeg` + range support; Web Audio sniffs the container, so the URL is all the same to it.
+// A host that does not implement /media/ keeps working: `audio.js` falls back to the original URL on 404.
+
+/** Prefix of the extension-less audio route served by `server/index.js`. */
+export const MEDIA_PREFIX = '/media/';
+
+/** Extensions `/media/…` may resolve to, in the order the server tries them. */
+export const AUDIO_EXTS = Object.freeze(['.mp3', '.m4a', '.aac', '.ogg', '.oga', '.opus', '.wav']);
+
+const AUDIO_PATH = /^\/assets\/audio\/(.+)$/i;
+
+/**
+ * Rewrite an audio URL to its extension-less `/media/…` form.
+ *
+ * Returns the input unchanged when the URL is not one of our audio assets, points at another origin (a CDN
+ * serves media itself, so the download-manager problem is not ours to solve there), or would produce a path the
+ * server refuses (empty / dot segments).
+ * @param {string} url
+ * @param {string} [origin] origin of the page; defaults to `location.origin`
+ * @returns {string} a URL safe to pass to fetch()
+ */
+export function mediaUrl(url, origin = globalThis.location?.origin) {
+  if (typeof url !== 'string' || !url) return url;
+  let u;
+  // No origin known (Node / tests): parse relative URLs against a placeholder, absolute ones are rejected below.
+  try { u = new URL(url, origin || 'http://localhost'); } catch { return url; }
+  // Cross-origin (or absolute while the page origin is unknown) stays untouched: that host serves its own media.
+  const absolute = /^[a-z][a-z0-9+.-]*:/i.test(url) || url.startsWith('//');
+  if (origin ? u.origin !== origin : absolute) return url;
+  const m = AUDIO_PATH.exec(u.pathname);
+  if (!m) return url;
+  let rest = m[1];
+  const ext = AUDIO_EXTS.find((e) => rest.toLowerCase().endsWith(e));
+  if (!ext) return url;             // not a file we serve as audio (e.g. "x.mp3.bak") — leave it alone
+  rest = rest.slice(0, -ext.length);
+  const segments = rest.split('/');
+  if (!rest || segments.some((s) => !s || s === '.' || s === '..' || s.startsWith('.'))) return url;
+  return `${MEDIA_PREFIX}${rest}${u.search}`;
+}
+
+/**
+ * Is this URL already one of the extension-less media paths?
+ * @param {string} url
+ * @param {string} [origin]
+ */
+export function isMediaUrl(url, origin = globalThis.location?.origin) {
+  return mediaUrl(url, origin) !== url;
+}