Grouped table

Grouping is a render-time arrangement, like the layout. Add group_by: :publisher — the gem orders by the group key, splits the rows into collapsible groups (chevron in the header) and shows a per-group count. With 29 books, groups start collapsed beyond config.group_collapse_threshold (default 50), opening only the first; the open ones live in ?open=, so a half-expanded view is copy-pasteable.

<%= crud_collection @books, group_by: :publisher %>   <%# belongs_to, enum or a column %>

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