Add support for interaction `i` transform (ala `fixest::i()`)#265

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matthewwardrop wants to merge 3 commits intomainfrom

add_interaction_operator

Commits on Jan 9, 2026

stateful_transforms: Add support for referencing active materializer.
matthewwardrop
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Add support for forcing categorical encoding to assume reduced rank.
matthewwardrop
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wip: Initial interaction (`i`) operator.
matthewwardrop
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