Update test suite for v2.0 and achieve >90% coverage

[jgallowa07]

Comprehensively update the test suite to validate v2.0 functionality and achieve >90% code coverage.

Tasks

• Run existing test suite: pytest tests/
• Document which tests fail and why
• Fix or remove tests that no longer apply
• Add tests for new v2.0 functionality
• Achieve >90% code coverage
• Add regression tests comparing v1.x and v2.0 results
• Ensure all doctests pass: pytest --doctest-modules -vv

Acceptance Criteria

• All tests pass
• Code coverage >90%
• Regression tests verify v2.0 ≈ v1.x results (within tolerance)
• CI passes on Python 3.9, 3.10, 3.11
• CI passes on Ubuntu and macOS