ibger 0.1.0

• Initial CRAN release.
• Core functions: ibge_aggregates(), ibge_metadata(), ibge_periods(), ibge_localities(), ibge_variables().
• Pre-flight validation against aggregate metadata for all query parameters.
• In-memory metadata cache with ibge_clear_cache().
• parse_ibge_value() utility for converting IBGE special value codes.
• parse_sidra_url() and fetch_sidra_url() for migrating from SIDRA API URLs (e.g. from the Query Builder or sidrar package).
• Support for flat view (view = "flat") response parsing.
• Four vignettes: getting started, API concepts, IPCA example, and a real-world tutorial on tracking state GDP components.