Implement harmonize_df()

[jonthegeek]

Closes #5.

[Copilot]

Pull Request Overview

This PR implements the harmonize_df() function to harmonize data frames according to specifications, completing issue #5. The implementation includes comprehensive validation, custom harmonization support, and flexible column handling.

Key changes:

• Added core harmonize_df() function with supporting internal helpers
• Refactored function naming conventions to follow .stop_if_*() pattern
• Enhanced specification validation with .to_hrmn_spec() utility
• Updated error class names for consistency

Reviewed Changes

Copilot reviewed 32 out of 32 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
R/harmonize_df.R	Core implementation of harmonize_df() and internal helpers
R/to_hrmn_spec.R	New specification validation utility functions
R/harmonize_fct.R	Updated to use new validation and naming conventions
R/specify_df.R	Updated to use renamed validation functions
R/aaa-conditions.R	Refactored helper functions with consistent naming
tests/testthat/test-harmonize_df.R	Comprehensive test suite for new functionality
tests/testthat/test-to_hrmn_spec.R	Tests for specification validation
man/*.Rd	Generated documentation for new functions
principles.md	Added design principles documentation

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