.Page.Store.Add is broken in Hugo 0.158+ (returns nil).
Additionally, Hugo renders shortcodes bottom-up, so the parent
tabs.html cannot pass data to child tab.html via .Store.
Fix:
- shortcode-id partial: replace .Page.Store.Add with explicit
.Page.Store.Set + add, which works in Hugo 0.161+.
- tab.html: the first child tab generates a page-unique ID via
the partial and caches it on .Parent.Store for siblings.
- tabs.html: remove broken .Store.Set "id" line.
This avoids the .Parent.Ordinal collision when tabs are nested
inside other shortcodes (details, columns, etc.).
Fixes#817
Daniel Strobusch