Implement records mode (long-format view) when drilling into aggregated cells.
Key changes:
- DrillIntoCell now creates views with _Index on Row and _Dim on Column
- GridLayout detects records mode and builds a records list instead of
cross-product row/col items
- Added records_display() to render individual cell values in records mode
- GridWidget and CSV export updated to handle records mode rendering
- category_names() now includes virtual categories (_Index, _Dim)
- Tests updated to reflect virtual categories counting toward limits
Co-Authored-By: fiddlerwoaroof/git-smart-commit (unsloth/Qwen3.5-35B-A3B-GGUF:Q5_K_M)
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