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/game-distribution/gameDistribution.css

@@ -494,7 +494,9 @@
   display: flex;
   flex-direction: column;
   gap: 0.9rem;
-  min-height: 100%;
+  /* 游玩页是固定画布：撑满面板而不是撑高面板，页面本身永不滚动。 */
+  height: 100%;
+  min-height: 0;
   padding: clamp(0.7rem, 1.8vw, 1.5rem);
 }
 .game-play-toolbar {
@@ -536,8 +538,8 @@
 }
 .game-player-shell {
   position: relative;
-  flex: 1;
-  min-height: min(72vh, 48rem);
+  flex: 1 1 auto;
+  min-height: 0;
   overflow: hidden;
   border: 1px solid rgba(199, 101, 61, 0.24);
   border-radius: 1.5rem;
@@ -581,7 +583,7 @@
   display: block;
   width: 100%;
   height: 100%;
-  min-height: min(72vh, 48rem);
+  min-height: 0;
   border: 0;
   opacity: 0;
   transition: opacity 220ms ease;
@@ -716,9 +718,34 @@
   .game-play-toolbar__actions .game-play-device-hint {
     display: none;
   }
-  .game-player-shell,
-  .game-player-frame {
-    min-height: calc(100dvh - 8rem);
+}
+
+/*
+ * 矮视口（横屏手机）里播放器只剩很小的高度，压扁启动面板保证
+ * 「开始游戏」可达；页面依旧不滚动，只有这一块在极端情况下内部兜底。
+ */
+@media (max-height: 26rem) {
+  .game-player-launch {
+    grid-template-columns: minmax(0, 1fr);
+    align-content: center;
+    gap: 0.5rem;
+    padding: 0.75rem 1rem;
+    text-align: center;
+    overflow-y: auto;
+  }
+  .game-player-launch .game-cover {
+    display: none;
+  }
+  .game-player-launch__copy p {
+    display: -webkit-box;
+    margin: 0.25rem 0 0.6rem;
+    overflow: hidden;
+    -webkit-box-orient: vertical;
+    -webkit-line-clamp: 2;
+  }
+  .game-player-loading,
+  .game-player-timeout {
+    font-size: 0.8rem;
   }
 }
 

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([]);

src/components/platform-entry/PlatformEntryActiveFlowShell.tsx

@@ -540,6 +540,11 @@ export function PlatformEntryFlowShellImpl({
     selectionStage === 'game-play' ||
     selectionStage === 'game-mine' ||
     selectionStage === 'game-publish';
+  /**
+   * 游玩页自己撑满剩余高度，面板不能再滚动，否则移动端会出现整页可滚的
+   * 半屏播放器；其余舞台继续沿用面板滚动。
+   */
+  const isGamePlayStage = selectionStage === 'game-play';
   const gameSearchParams = new URLSearchParams(
     typeof window === 'undefined' ? '' : window.location.search,
   );
@@ -717,7 +722,7 @@ export function PlatformEntryFlowShellImpl({
 
               <main
                 id={isCreationStage ? 'platform-tab-panel-create' : undefined}
-                className={`platform-tab-panel min-h-0 min-w-0 flex-1 overflow-auto ${isCreationStage || isProfileStage ? '' : 'px-3 py-4 sm:px-6 sm:py-6'}`}
+                className={`platform-tab-panel min-h-0 min-w-0 flex-1 ${isGamePlayStage ? 'platform-tab-panel--game-play' : 'overflow-auto'} ${isCreationStage || isProfileStage || isGamePlayStage ? '' : 'px-3 py-4 sm:px-6 sm:py-6'}`}
               >
                 {isProfileStage ? (
                   <PlatformActiveProfileView