When a formula Ref resolves to an item that doesn't exist in any
category, find_item_category returned None and the fallback
unwrap_or(name) used the item name as a category name. The resulting
key still matched the formula target, causing infinite recursion and a
stack overflow. Now returns None immediately via ? so the expression
evaluates to Empty.
Adds unit tests for Model (structure, cell I/O, views), formula
evaluation (arithmetic, WHERE, aggregation, IF), and a full
five-category Region×Product×Channel×Time×Measure integration test.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Ed L
6a9d28ecd6