* fix(tabs): fix showMoreTabs overflow calculation lag caused by CSS transition
The nav element has CSS transition: transform 0.3s, which causes
getBoundingClientRect() to return pre-transition positions when
calcMorePanes runs in requestAnimationFrame (before the transition
has advanced).
Fix: use tab-relative-to-nav position delta (transition-invariant
since both share the same transform coordinate space) plus navOffset
to compute the final expected positions, bypassing the transition
intermediate state entirely.
Also replace nextTick with requestAnimationFrame in deferred
calcMorePanes calls to ensure all cascading DOM updates from
scrollToActiveTab -> updated() chains have settled.
* fix(tab-nav): trigger calcMorePanes after scrollPrev/scrollNext arrow click
* test(tabs): 修正 custom-more-icon 溢出下拉数量断言
showMoreTabs 溢出计算修正后按真实布局计算，容器 400px 减去
更多按钮占位后仅 5 个 tab 完整可见，下拉应为 Tab 6/7/8 共 3 项，
原断言 2 是基于旧实现 mount 早期量宽了 navScroll 的错误结果。
fdb2129f86