fix(skeleton): put the widget gutter relief on the skeleton's inner item

小工具（首页挂载的 / 侧栏的）骨架加载占位比真实卡片窄一截：card 布局下
每侧窄 8px，视觉上就是「骨架小了一些」。

小工具的卡片走 skip_wrapper=true，直接挂在 .widget-ajaxpager 上
（src/WidgetSystem/PostRenderer.php），不经过 .posts-row，因此只受小工具
这条独立的负边距链作用：

  .widget-ajaxpager            margin: -8px
  .posts-item.card（真实卡片）  margin: 8px   ← 卡片自身自带
  .post_ajax_loader（外层）     margin: 8px   ← f61142ee 加的补偿
  .posts-item.card（骨架内层）  margin: 8px   ← 同一条卡片规则

骨架于是拿到 8px（外层 loader）+ 8px（内层卡片）= 16px，而真实卡片只有
8px，内容区每侧少 8px。list 模式没暴露是因为 .posts-item 的 8px 来自
`.widget-main-post.style-list .posts-item`，该规则只覆盖 style-list、
不作用于骨架，外层的 8px 恰好补上 list 卡片应有的一份，刚好对齐。

修法：把水平补偿从外层 .post_ajax_loader 移到骨架内层的 .posts-item /
.posts-mini —— 与真实卡片同一个元素、同一条选择器，于是两者逐像素同宽，
也顺带回答了「补偿链的拥有者是谁」这个一直在打补丁的问题。

- assets/css/main.css、assets/css/frontend/post-card.css：.posts-row 与
  .widget-ajaxpager 两条链、桌面 8px 与移动 5px 两组，共 8 处规则改为
  内层选择器。
- assets/css/routes/user-home.css：面板归零规则同步改到内层，否则它归零
  的是已经失效的外层，旧补偿会重新显形。
- tests/widget-system/asset-ledger-contract.test.cjs：把守卫断言改成内层
  选择器，并修正规则匹配以支持选择器组（.posts-item, .posts-mini）。
  注释说明为何不能补偿外层。

参考主题 zibll 完全没有 .post_ajax_loader 的 margin 规则；AKSRT 需要它，
是因为骨架多包了一层。定位在内层是与参考主题等价的正确解。

Verification:
- node tests/widget-system/asset-ledger-contract.test.cjs  16/16
- node tests/widget-system/post-visual-contract.test.cjs   22/22
- node tests/widget-system/zibll-modern-widgets-contract.test.cjs 10/10
- node tests/widget-system/lazyload-contract.test.cjs      7/7
- node tests/widget-system/frontend-lifecycle-contract.test.cjs 13/13
- node tests/widget-system/page-display-contract.test.cjs  18/18
- node tests/widget-system/message-center-contract.test.cjs 8/8
- php tools/audit-admin-contracts.php  errors=0
- build:public 已重建；旧外层规则在源与产物中均已归零

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/widget-system/asset-ledger-contract.test.cjs

@@ -97,26 +97,33 @@ test('skeleton blocks share the card gutter instead of overflowing it', () => {
   // card a matching gutter. The card rule is scoped
   // `.posts-row > *:not(.post_ajax_loader)`, so the loader is excluded from the
   // compensation chain and ends up wider than the cards by both gutters.
-  // Each gutter block therefore needs a sibling rule that gives the loader the
-  // same horizontal margin — otherwise 加载更多 shows a skeleton that spills
-  // outside the content column.
+  //
+  // The relief must land on the skeleton's INNER `.posts-item` — the same
+  // element and selector the real card uses — never on the outer
+  // `.post_ajax_loader`. A block skeleton is `> .post_ajax_loader > .mb20 >
+  // .posts-item.card`, and `.posts-item.card` already carries margin:8px of its
+  // own: compensating the outer wrapper too would stack 8px + 8px and make the
+  // card skeleton 8px narrower per side than a real card (the widget card
+  // layout, which skips .posts-row and relies on the .widget-ajaxpager chain).
   const blocks = [
     {
       file: 'assets/css/main.css',
-      selector: '.posts-row > .post_ajax_loader',
+      selector: '.posts-row > .post_ajax_loader .posts-item',
       gutters: [8, 5],
     },
     {
       file: 'assets/css/frontend/post-card.css',
-      selector: '.posts-row > .post_ajax_loader',
+      selector: '.posts-row > .post_ajax_loader .posts-item',
       gutters: [8, 5],
     },
   ]
 
   for (const block of blocks) {
     const css = fs.readFileSync(path.join(root, block.file), 'utf8')
+    // The rule ships a selector group (`.posts-item, .posts-mini`), so match the
+    // whole group up to its `{` instead of assuming the selector is alone.
     const rules = (css.match(
-      new RegExp(`${block.selector.replace(/[.*+?^${}()|[\]\\]/g, '\\$&')}\\s*\\{[^}]*\\}`, 'g')
+      new RegExp(`${block.selector.replace(/[.*+?^${}()|[\]\\]/g, '\\$&')}[^{]*\\{[^}]*\\}`, 'g')
     ) || []).map((rule) => rule.replace(/\s+/g, ' '))
 
     // One rule per gutter value in that file (desktop 8px, mobile 5px).
@@ -128,26 +135,27 @@ test('skeleton blocks share the card gutter instead of overflowing it', () => {
       )
       assert.ok(
         matched,
-        `${block.file} must give .post_ajax_loader the ${gutter}px gutter in "${block.selector}"`
+        `${block.file} must give .post_ajax_loader .posts-item the ${gutter}px gutter in "${block.selector}"`
       )
     }
   }
 
   // The widget pagers apply their own negative-margin compensation chain
-  // (.widget-ajaxpager / .tab-pane), so the loader needs the same relief there.
+  // (.widget-ajaxpager / .tab-pane), so the skeleton's inner item needs the same
+  // relief there — again on .posts-item, not on the loader wrapper.
   for (const file of ['assets/css/main.css', 'assets/css/frontend/post-card.css']) {
     const css = fs.readFileSync(path.join(root, file), 'utf8')
     // Cover every style class the renderer can emit (.style-list / .style-card,
     // and the tab pane which carries style-list or index-tab).
-    assert.match(css, /\.widget-main-post\.style-list \.widget-ajaxpager > \.post_ajax_loader/)
-    assert.match(css, /\.widget-main-post\.style-card \.widget-ajaxpager > \.post_ajax_loader/)
-    assert.match(css, /\.widget-tab-post \.tab-content > \.tab-pane > \.widget-ajaxpager > \.post_ajax_loader/)
+    assert.match(css, /\.widget-main-post\.style-list \.widget-ajaxpager > \.post_ajax_loader \.posts-item/)
+    assert.match(css, /\.widget-main-post\.style-card \.widget-ajaxpager > \.post_ajax_loader \.posts-item/)
+    assert.match(css, /\.widget-tab-post \.tab-content > \.tab-pane > \.widget-ajaxpager > \.post_ajax_loader \.posts-item/)
   }
 
   // The user-home panel zeroes the container margin and the card gutters, so the
-  // loader must not pick up any leftover compensation there.
+  // skeleton's inner item must not pick up any leftover compensation there.
   const userHome = fs.readFileSync(path.join(root, 'assets/css/routes/user-home.css'), 'utf8')
-  assert.match(userHome, /\.aksrt-user-home__post-list\[data-view="list"\] > \.post_ajax_loader \{\s*margin-right: 0;\s*margin-left: 0;/)
+  assert.match(userHome, /\.aksrt-user-home__post-list\[data-view="list"\] > \.post_ajax_loader \.posts-item \{\s*margin-right: 0;\s*margin-left: 0;/)
 })
 
 test('asset tables are InnoDB and operation keys are unique', () => {