游玩页改成固定画布，移动端不再整页可滚

- PlatformEntryActiveFlowShell 在 game-play 舞台给页签容器加 platform-tab-panel--game-play，不再挂 overflow-auto，也不再叠加双份内边距
- index.css 新增 .platform-tab-panel--game-play，面板 overflow:hidden 并去掉右侧留白
- gameDistribution.css 让 .game-play-page 改 height:100%/min-height:0、播放器 flex:1 1 auto/min-height:0，删除移动端把页面撑高的 calc(100dvh - 8rem)
- 矮视口（max-height:26rem，横屏手机）压扁启动面板并隐藏封面，保证「开始游戏」可达，页面与面板本身仍不滚动
- PlatformEntryActiveFlowShell.test.tsx 双向锁定游玩舞台页签容器不滚动、游戏广场等舞台仍保持滚动
- check-game-distribution-web-e2e.mjs 移动端段新增「移动端游玩页不可滚动（面板与整页都不溢出）」断言
- 里程碑文档补上该修复的根因、口径与验证记录

src/components/platform-entry/PlatformEntryActiveFlowShell.test.tsx

@@ -898,6 +898,35 @@ describe('游戏发布灰度入口', () => {
     );
   });
 
+  it('游玩舞台的页签容器不滚动', async () => {
+    gameDistributionMock.getGame.mockResolvedValue(null);
+
+    const { container } = render(
+      <StatefulPlatformEntryFlowShell initialStage="game-play" />,
+    );
+
+    const panel = container.querySelector('.platform-tab-panel');
+    expect(panel?.classList.contains('platform-tab-panel--game-play')).toBe(
+      true,
+    );
+    expect(panel?.className).not.toContain('overflow-auto');
+  });
+
+  it('游戏广场等舞台仍保持页签容器滚动', async () => {
+    gameDistributionMock.listGames.mockResolvedValue([]);
+
+    const { container } = render(
+      <StatefulPlatformEntryFlowShell initialStage="games" />,
+    );
+    expect(await screen.findByText('游戏广场')).toBeTruthy();
+
+    const panel = container.querySelector('.platform-tab-panel');
+    expect(panel?.classList.contains('platform-tab-panel--game-play')).toBe(
+      false,
+    );
+    expect(panel?.className).toContain('overflow-auto');
+  });
+
   it('灰度命中时广场展示发布入口', async () => {
     loginAsAuthor();
     gameDistributionMock.listGames.mockResolvedValue([]);