refactor(test): consolidate duplicate test logic in diff_test.go

Eliminates dupl warning by refactoring TestDiffResult_IsImprovement
and TestDiffResult_IsRegression into a single table-driven test.

- Combines duplicate test cases into TestDiffResult_Evaluation
- Uses table-driven test pattern for better maintainability
- Preserves all original test coverage
- Reduces code duplication by ~50 lines

Fixes: golangci-lint dupl warning