fix(single): place the video footer breadcrumb like the reference

Drop the invented .post-breadcrumbs wrapper and echo the breadcrumb
inside the reference's flex jsb at row; split .single-video .breadcrumb
out of the cover-overlay rule (-6px pull and opacity reset belong to
title-over-cover contexts only) and restore the legacy 15px footer
padding. Pin the structure in the cover contract test and rebuild the
homepage aggregates.

assets/css/frontend/single-post/cover.css

@@ -50,13 +50,6 @@
     background: var(--color-bg-2, #f5f5f5);
 }
 
-/* ===== 面包屑 ===== */
-.post-breadcrumbs {
-    margin-bottom: 12px;
-    font-size: 13px;
-    color: var(--color-ink-4, var(--color-ink-4));
-}
-
 .article-title + .img-badge,
 .single-head-cover .img-badge.right,
 .article-header .img-badge.right {
@@ -101,7 +94,8 @@
 
 
 /* ===== Video footer (legacy single-video-footer) ===== */
-.single-head-cover .single-video-footer { padding: 12px 4px 4px; }
+/* 内边距沿用 legacy 契约 .single-video-footer{padding:15px}（与 zibll 一致），
+   面包屑在 flex jsb at 行首，不再额外包装。 */
 .single-head-cover .single-video-footer .article-title {
     font-size: clamp(1.1rem, 2.4vw, 1.45rem); font-weight: 700; margin: 8px 0 0; line-height: 1.35;
 }