Add property tests and CI with coverage

[farukalpay]

Summary

• add property-based tests for gradient descent and graph invariants
• configure coverage and omit heavy modules for focused reporting
• set up GitHub Actions CI with lint, type-check and multi-platform test matrix
• document CI and coverage status badges in README

Testing

• pre-commit run --files README.md pyproject.toml requirements.txt tests/test_algorithms_property.py tests/test_graph_properties.py .github/workflows/ci.yml .coveragerc
• pytest --cov=src --cov-report=term-missing --cov-fail-under=90

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https://chatgpt.com/codex/tasks/task_e_68aa20193af083238ed7437fef0653da