# Formula Function Expansion

The formula engine is now separate from the grid model and includes a broader Excel/Google Sheets function foundation.

## New Function Families

- Conditional aggregation: `SUMIF`, `COUNTIF`, `AVERAGEIF`
- Error handling: `IFERROR`
- Lookup: `HLOOKUP`, `XLOOKUP`, `LOOKUP`
- Conversion / text: `VALUE`, `TEXT`
- Date helpers: `DATEVALUE`, `EOMONTH`
- Dynamic-array foundation helpers: `UNIQUE`, `FILTER`

## Design Notes

The formula engine still uses a safe parser/evaluator approach. Unsupported formula syntax returns error values rather
than executing arbitrary JavaScript. Array/spill behavior is represented as joined values for now; a future spill-grid
engine can replace that seam without changing the public `evaluateFormula` entry point.