fix(single): stop the video cover letterboxing inside its ratio box

The player init wrote an inline object-fit:contain on the video
element, which outranked the injected stylesheet's object-fit:cover
fill contract and left black bars above/below whenever the cover box
ratio differed from the video. Drop the inline style so the
stylesheet rule is the single source of truth, and pin the
no-inline-style rule in the cover contract test.

tests/widget-system/single-cover-height-contract.test.cjs

@@ -99,6 +99,11 @@ test('video cover: the player video fills and center-crops like the image cover'
     /max-height:70vh/,
     '封面盒高度由比例盒决定，不得再套 70vh 截断'
   );
+  assert.doesNotMatch(
+    player,
+    /video\.style\.cssText/,
+    '视频不得携带内联样式：内联 object-fit 会以更高优先级覆盖注入样式表的 cover 契约，产生信箱式黑边'
+  );
 });
 
 test('video cover: play never runs against a sourceless player', () => {