Test: add unit tests co-located with the code they cover
- model/cell.rs: CellKey (sorting, get, with, without, matches_partial,
display) and DataStore (set/get, overwrite, empty-removes-key,
sum_matching, matching_cells, text exclusion)
- model/category.rs: item ids, deduplication, group assignment,
top_level_groups, item_index insertion order
- formula/parser.rs: subtraction, WHERE clause, SUM/AVG, IF,
numeric literal, chained arithmetic, error on missing =
- view/view.rs: auto-axis assignment, set_axis, categories_on,
page_selection, group collapse toggle, hide/show, cycle_axis
(all transitions + scroll/selection reset)
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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