restrictR 0.1.0

• Initial release.
• Core constructor: restrict() for building composable validators.
• Building blocks: require_df(), require_numeric(), require_character(), require_length(), require_length_matches(), require_nrow_min(), require_has_cols(), require_col_numeric(), require_col_character(), require_col_range(), require_range(), require_one_of().
• Dependent validation via one-sided formulas with explicit context passing.
• Path-aware error messages (e.g. newdata$x2 must be numeric).
• as_contract_text() for roxygen-compatible documentation.