Reorganise tutorials and improve symbolic matrix helpers

[YeeShin504]

Summary

This branch cleans up the public API, improves symbolic matrix rendering, and expands the tutorial/test layout for broader test coverage.

Highlights

• Moved sympy_commands() out of symbolic.py into utils.py.
• Tightened custom type behavior and coverage, including RREFCase, PDP, SVD, QR, and NumSVD.
• Reorganised chapter and tutorial tests to better match the documentation structure.
• Updated tutorial notebooks to use the updated evaluate_cases and new rref_cases method.

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Codecov Report

✅ All modified and coverable lines are covered by tests.

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