[CultInspectorGroupBy(nameof(A), nameof(B))] on a document class names the
members its record list groups by, outermost first. CultInspectorModel owns
the rest: GroupingOf validates the declaration (a refused one gives a flat
list and a notice), GroupRecords partitions RecordCandidates by value
identity with occupied values only, CreateInGroup builds the record a node
holds, RecordRefLabel owns None and Missing <key>. A variant groups by its
resolved Document.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
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