fix(single): fill slide and video cover media like the image cover

Slide-mode images rendered at intrinsic ratio, top-aligned inside the
swiper chain (--pc-height:auto left .swiper-wrapper content-sized), so
the taller box showed a gap at the bottom and never center-cropped like
the image cover. The video cover had the same problem through
video-player.js (height:auto, capped at 70vh).

- aksrt_render_slider scale mode emits --pc-height/--m-height 100% so
  the wrapper/slide/img chain fills the ratio box and object-fit: cover
  center-crops inside it (same pattern as the .scale-height widgets)
- video-player.js renders the video at height:100% + object-fit:cover;
  with an undimensioned container height:100% degrades to auto,
  matching the previous behavior
- drop the dead .single-cover padding-bottom rules (every producer owns
  its height inline, and the rule squeezed the absolutely positioned
  slide inner) and the orphaned --single-cover-scale var in
  tokens.css/compat-vars.css/main.css/InlineStyleBuilder
- rebuild public bundles; extend the single-cover-height contract

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

@@ -68,14 +68,66 @@ test('single cover: the dead static slide fallback is gone', () => {
   );
 });
 
-test('slider scale mode: inner swiper carries no dead inline height styles', () => {
+test('slider scale mode: completes the definite height chain for cover cropping', () => {
   assert.doesNotMatch(
     sliderHelper,
     /--scale-ratio/,
     '--scale-ratio 无任何 CSS 消费方，禁止输出'
   );
-  const scaleMode = sliderHelper.split("if ($args['scale_height']) {")[1] || '';
-  assert.ok(scaleMode.includes("position:absolute;inset:0;width:100%;height:100%;"));
+  // wrapper 决定盒子高度；pc/m-height 100% 让 .swiper-wrapper/.swiper-slide
+  // 铺满比例盒，使 object-fit: cover 在确定的盒内居中裁切（对齐单图封面）。
+  assert.match(
+    sliderHelper,
+    /\$height_style = '--pc-height:100%;--m-height:100%;';/
+  );
+  assert.match(
+    sliderHelper,
+    /esc_attr\(\$height_style \. \(\$args\['scale_height'\] \? 'position:absolute;inset:0;width:100%;height:100%;' : ''\)\)/,
+    '内层 .new-swiper 必须绝对定位铺满 wrapper'
+  );
+});
+
+test('video cover: the player video fills and center-crops like the image cover', () => {
+  const player = read('assets/js/video-player.js');
+  assert.match(
+    player,
+    /\.aksrt-video video\{display:block;width:100%;height:100%;object-fit:cover;\}/,
+    '视频必须铺满封面比例盒并居中裁切'
+  );
+  assert.doesNotMatch(
+    player,
+    /max-height:70vh/,
+    '封面盒高度由比例盒决定，不得再套 70vh 截断'
+  );
+});
+
+test('single cover css owns no fallback padding and the scale var is gone', () => {
+  for (const file of [
+    'assets/css/frontend/single-post/cover.css',
+    'assets/css/main.css',
+    'assets/css/main.min.css',
+    'assets/css/core/tokens.css',
+    'assets/css/frontend/compat-vars.css',
+    'assets/css/build/public.css',
+    'assets/css/build/public.min.css',
+  ]) {
+    const css = read(file);
+    assert.doesNotMatch(
+      css,
+      /\.single-cover\s*\{[^}]*padding-bottom/,
+      `${file} 的 .single-cover 不得声明 padding-bottom（高度归内联样式）`
+    );
+    assert.doesNotMatch(
+      css,
+      /--single-cover-scale/,
+      `${file} 不得再包含孤儿变量 --single-cover-scale`
+    );
+  }
+  assert.doesNotMatch(
+    read('src/Frontend/InlineStyleBuilder.php'),
+    /--single-cover-scale/,
+    '运行时 CSS 变量不得再输出孤儿 --single-cover-scale'
+  );
 });
 
 test('dead cover css is removed from every stylesheet surface', () => {