fix(ajax): show the reference skeleton on every load-more surface

Clicking 加载更多 in the 多栏目文章 widget showed no skeleton at all — the
list simply stayed still until the response arrived, then the new cards
appeared. The homepage stream, archives and comments already had a working
skeleton; the widget's AJAX load goes through a different client and never
found its loader.

Root cause was DOM placement, not visibility: ContentRenderer emitted
.post_ajax_loader as a *sibling* of .widget-ajaxpager, while the widget REST
client resolves it with container.querySelector('.post_ajax_loader'). The
node existed but could never match, so .is-loading was never added and the
skeleton stayed display:none. Inactive tab panes had the same shape, and the
direct-load branch of ajaxpager() emitted no loader at all.

Changes, all following the reference theme's post_ajax() contract in
zibll/js/main.js (beforeSend anchors the loader to the end of the list;
success only hides it, never removes it):

- ContentRenderer: nest the loader inside .widget-ajaxpager for both the
  direct-load and inactive-tab branches, and drop the sibling append.
- widgets/frontend.js: anchorLoader() moves the loader to the end of the
  list before .ajax-pag on every request; appended cards are inserted before
  it; a replace preserves the node instead of wiping it with innerHTML; a
  fallback loader is created when the DOM has none; loadItems() now returns
  its promise; drop the unreachable is-loading auto-load branch.
- Message centre, user-home panels and the search page switch from spinner /
  opacity feedback to the same .posts-item-shaped skeleton, so all AJAX
  surfaces share one loading contract.
- Remove the second, never-wired skeleton system (.aksrt-skeleton-* in
  core-ui.css) and the dead spinner stylesheets/rules (skeleton.css,
  ajax-spinner.css, .load-more__*, --skeleton-base/--skeleton-shine) plus the
  unregistered assets/js/waterfall.js.
- Extend the widget, message-centre, user-home and asset-ledger contract
  tests to pin the loader inside the container, the class-only visibility
  switch, and the absence of the deleted dead code.

Verified: 470 node tests, 136 PHP widget assertions, admin-contract and
widget-system audits all green; rebuilt public assets after the CSS/JS
removals.

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

assets/css/frontend/enhancements/core-ui.css

@@ -1,8 +1,11 @@
 /**
  * AKSRT Enhancements — 全局 UI 增强
  *
- * 包含：骨架屏、悬停动效、暗色过渡、滚动感知
+ * 包含：悬停动效、暗色过渡、滚动感知
  * 所有选择器使用 aksrt- 前缀避免冲突
+ *
+ * 骨架屏不在此文件：唯一契约是 .placeholder 块 + .post_ajax_loader 容器
+ * （inc/helpers/posts-renderer/markup.php 的 aksrt_render_placeholder）。
  */
 
 @keyframes aksrt-spin {
@@ -11,79 +14,7 @@
 }
 
 /* ============================================================
- * 1. 骨架屏加载动画
- * ============================================================ */
-.aksrt-skeleton {
-    background: linear-gradient(
-        90deg,
-        rgba(121, 121, 121, 0.08) 25%,
-        rgba(121, 121, 121, 0.15) 37%,
-        rgba(121, 121, 121, 0.08) 63%
-    );
-    background-size: 400% 100%;
-    animation: aksrt-skeleton-shimmer 1.5s ease infinite;
-    border-radius: 4px;
-}
-
-.aksrt-skeleton--image {
-    height: 0;
-    padding-bottom: 66.67%; /* 3:2 aspect ratio */
-}
-
-.aksrt-skeleton--avatar {
-    width: 40px;
-    height: 40px;
-    border-radius: 50%;
-}
-
-.aksrt-skeleton--title {
-    height: 22px;
-    width: 80%;
-}
-
-.aksrt-skeleton--text {
-    height: 16px;
-    width: 100%;
-}
-
-.aksrt-skeleton--text-short {
-    height: 16px;
-    width: 60%;
-}
-
-.aksrt-skeleton--button {
-    height: 36px;
-    width: 100px;
-    border-radius: 8px;
-}
-
-@keyframes aksrt-skeleton-shimmer {
-    0% { background-position: 100% 50%; }
-    100% { background-position: 0 50%; }
-}
-
-/* 骨架屏组合：文章卡片骨架 */
-.aksrt-skeleton-card {
-    background: var(--surface-1, #fff);
-    border: 1px solid var(--color-line);
-    border-radius: 12px;
-    overflow: hidden;
-    padding: 0;
-}
-
-.aksrt-skeleton-card .aksrt-skeleton--image {
-    border-radius: 0;
-}
-
-.aksrt-skeleton-card__body {
-    padding: 16px;
-    display: flex;
-    flex-direction: column;
-    gap: 10px;
-}
-
-/* ============================================================
- * 2. 悬停交互动效
+ * 1. 悬停交互动效
  * ============================================================ */
 
 /* 卡片放大 */