Spreadsheet Engine

# Spreadsheet Engine PRD

## Overview
Build a functional spreadsheet engine with a web-based UI that supports formulas, cell references, and real-time recalculation.

## Requirements
- Grid of cells (minimum 26 columns A-Z, 100 rows)
- Cell editing: click to select, type to edit, Enter to confirm, Escape to cancel
- Data types: numbers, strings, formulas (start with =)
- Formula evaluation with cell references: =A1+B2, =A1*2+1
- Range references: =SUM(A1:A10), =AVERAGE(B1:B5)
- Built-in functions: SUM, AVERAGE, MIN, MAX, COUNT, IF, CONCAT
- Dependency graph: changing A1 recalculates all cells that reference A1
- Circular reference detection with error display
- Cell formatting: bold, italic, text alignment
- Column resizing by dragging
- Row and column headers (A, B, C... and 1, 2, 3...)
- Keyboard navigation: arrow keys, Tab, Shift+Tab
- Copy/paste cells (Ctrl+C, Ctrl+V)
- Undo/redo (Ctrl+Z, Ctrl+Y)
- Export to CSV

## Tech Stack
- Any frontend framework
- No spreadsheet libraries — build the engine yourself

## Scoring Criteria
- **Functional (40%)**: Cell editing, formulas, references, dependency recalculation, cycle detection
- **Quality (20%)**: Performance with 1000+ cells, no rendering glitches, clean architecture
- **Fidelity (25%)**: All features present, keyboard navigation works, export works
- **Speed (15%)**: Time bonus