Implemented SubtractSaturating method for TimeSpan extensions with comprehensive testing and XML documentation.

• Explored repository structure and understand codebase
• Built and tested existing code to confirm working state (550 tests pass)
• Analyzed existing SafeAdd/SafeSubtract patterns in DateTime extensions
• Implement SubtractSaturating method in TimeSpanExtensions
• Create comprehensive tests for the new method
• Verify implementation follows existing patterns and conventions
• Run all tests to ensure no regressions (558 tests pass)
• Manual verification with test program confirms correct behavior
• Add XML documentation to SubtractSaturating method
• Refactor tests to follow MethodName_StateUnderTest_ExpectedBehavior naming convention

The SubtractSaturating method subtracts one TimeSpan from another and ensures the result doesn't go below TimeSpan.Zero. This implements saturating arithmetic for TimeSpan subtraction, preventing negative results.

Key features:

• Returns self.Subtract(other) when self >= other
• Returns TimeSpan.Zero when result would be negative
• Handles edge cases including zero values and negative TimeSpan inputs
• Follows existing code patterns and naming conventions
• Comprehensive test coverage with 8 focused test methods following best practices
• Complete XML documentation following project conventions

Fixes #43.

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