Add QuintoAndar ImovelWeb Rental Index (IQAIW) dataset:
- New dataset covering 6 Brazilian cities (BHE, BSB, CUR, POA, RIO, SPO)
- Hedonic double imputed rental price index from 2023 onwards
- Variables: date, city, index, monthly change, 12-month change, price/m²

Documentation includes:
- Detailed methodology explanation in R/data.R
- Complete variable descriptions with units
- Historical context and comparison to former IQA index
- Example usage with benviplot functions
- Public data source URL

Data processing script (data-raw/iqaiw.R):
- Downloads from public QuintoAndar repository
- Validates data structure with informative errors
- Converts city codes to full names
- Calculates normalized index (base=100)
- Comprehensive comments explaining each step

R CMD check: 0 errors, 0 warnings, 0 notes ✓

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>