修复在线游玩加载体验：加载期黑屏/空白与发行资源传输开销

- packages/shared 新增共享游戏加载面 PlatformGameLoadingSurface：封面/标题 + 动效进度 + 按时长推进的阶段文案（准备/资源/引擎）+ 慢加载提示，附用例
- 网页游玩页用共享加载面替换近黑壳上的一行小字，加载上限改为共享常量 PLATFORM_GAME_LOADING_TIMEOUT_MS
- 后台审核试玩补加载面、20 秒超时与「重新创建试玩会话」入口，新会话重置加载态；补两条用例
- 发行网关（api-server）对文本类发行资源按 Accept-Encoding 下发 gzip，并下发强 ETag、命中 If-None-Match 返回 304；补 5 条 Rust 用例
- E2E 脚本 check-game-distribution-ops-rollback-e2e 补压缩/Vary/ETag/304/不接受 gzip 四条断言
- 同步 pitfalls、开发运维文档、nginx README 与两份里程碑记录

scripts/check-game-distribution-ops-rollback-e2e.mjs

@@ -18,6 +18,7 @@
 //   E2E_ADMIN_USER=... E2E_ADMIN_PASSWORD=... npm run check:game-distribution-ops-rollback-e2e
 import { createHash, randomBytes } from 'node:crypto';
 import { readdirSync, readFileSync, statSync } from 'node:fs';
+import http from 'node:http';
 import path from 'node:path';
 
 import JSZip from 'jszip';
@@ -99,6 +100,37 @@ async function release(pathname) {
   return { status: response.status, body, headers: response.headers };
 }
 
+/// 原样请求（不自动解压）：压缩与条件请求必须看真实线字节，不能让 fetch 帮忙解码。
+function releaseRaw(pathname, { acceptEncoding, ifNoneMatch } = {}) {
+  return new Promise((resolve, reject) => {
+    const target = new URL(`${API}${pathname}`);
+    const headers = {};
+    if (acceptEncoding) headers['accept-encoding'] = acceptEncoding;
+    if (ifNoneMatch) headers['if-none-match'] = ifNoneMatch;
+    const request = http.request(
+      {
+        hostname: target.hostname,
+        port: target.port,
+        path: `${target.pathname}${target.search}`,
+        headers,
+      },
+      (response) => {
+        const chunks = [];
+        response.on('data', (chunk) => chunks.push(chunk));
+        response.on('end', () =>
+          resolve({
+            status: response.statusCode,
+            headers: response.headers,
+            body: Buffer.concat(chunks),
+          }),
+        );
+      },
+    );
+    request.on('error', reject);
+    request.end();
+  });
+}
+
 async function adminToken() {
   const login = await api('/admin/api/login', {
     method: 'POST',
@@ -397,6 +429,47 @@ async function main() {
   );
   assertReleaseHeaders('v1 CSS', baselineCss, { html: false });
 
+  // 传输与条件请求契约：引擎包动辄 1 MiB 以上，文本资源必须按 Accept-Encoding 压缩，
+  // 并在 60 秒缓存窗口之后靠 ETag 重验证（304）而不是重下整包。
+  const releasePath = `/api/game-distribution/releases/${gameId}/index.html`;
+  const compressedHtml = await releaseRaw(releasePath, {
+    acceptEncoding: 'gzip',
+  });
+  check(
+    '发行文本资源按 Accept-Encoding 压缩并声明 Vary',
+    compressedHtml.status === 200 &&
+      compressedHtml.headers['content-encoding'] === 'gzip' &&
+      String(compressedHtml.headers.vary ?? '').includes('accept-encoding') &&
+      compressedHtml.body.length < Buffer.byteLength(baselineRelease.body),
+    `encoding=${compressedHtml.headers['content-encoding'] ?? ''} vary=${compressedHtml.headers.vary ?? ''} bytes=${compressedHtml.body.length}/${Buffer.byteLength(baselineRelease.body)}`,
+  );
+  const releaseEtag = baselineRelease.headers.get('etag') ?? '';
+  check(
+    '发行资源下发强 ETag',
+    /^"[0-9a-f]{32}"$/u.test(releaseEtag),
+    `etag=${releaseEtag}`,
+  );
+  const revalidated = await releaseRaw(releasePath, {
+    ifNoneMatch: releaseEtag,
+  });
+  check(
+    'If-None-Match 命中时返回 304 且不带正文',
+    revalidated.status === 304 &&
+      revalidated.body.length === 0 &&
+      revalidated.headers['cache-control'] === RELEASE_CACHE_CONTROL &&
+      revalidated.headers.etag === releaseEtag,
+    `status=${revalidated.status} body=${revalidated.body.length} cache-control=${revalidated.headers['cache-control'] ?? ''}`,
+  );
+  const identityHtml = await releaseRaw(releasePath, {
+    acceptEncoding: 'identity',
+  });
+  check(
+    '客户端不接受 gzip 时不下发压缩体',
+    identityHtml.status === 200 &&
+      identityHtml.headers['content-encoding'] === undefined,
+    `encoding=${identityHtml.headers['content-encoding'] ?? ''}`,
+  );
+
   const pendingIds = {};
   for (const marker of ['V2-OK', 'V3-OK']) {
     const built = await buildPackage(marker);