Complete TreeStructure class arithmetic member functions

The TreeStructure class currently supports the + operator for recursive tree merging. However, additional arithmetic and logical operators are stubbed as a TODO:

# TODO: add more arithmetic functions
# def __sub__(self) -> Self:
# def __and__(self) -> Self:
# def __or__(self) -> Self:
# def __xor__(self) -> Self:

These should be properly implemented to enhance the class's composability and utility in symbolic or config-tree manipulation contexts.

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Suggested Tasks

• Define __sub__ to recursively subtract or remove subtrees.
• Define __and__ for intersection of keys/values (e.g., common paths).
• Define __or__ for union (like + but with different conflict resolution).
• Define __xor__ for symmetric difference between tree keys/values.
• Add tests for all new operators.
• Document functions