- Add validation in Serie.__init__ to prevent empty/whitespace keys
- Add validation in Serie.key setter to prevent empty values
- Automatically strip whitespace from key values
- Add comprehensive docstrings explaining key as unique identifier
- Document folder property as metadata only (not for lookups)
- Create comprehensive test suite with 16 tests in test_serie_class.py
- All 56 Serie-related tests pass successfully
- Update instructions.md to mark Task 1.1 as completed
This is the first task in the Series Identifier Standardization effort
to establish 'key' as the single source of truth for series identification
throughout the codebase.
