Add sparse transition row cache

[FlorianPfaff]

Summary:

• add reusable sparse transition row cache utilities
• wire sparse second-order grid evidence through the cache
• document and cover cache behavior with focused regressions

Verification:

• python -m py_compile src\pyrecest\filters\sparse_transition_cache.py src\pyrecest\filters\sparse_second_order_grid.py tests\test_sparse_transition_cache.py
• git diff --check

Tests were not run per request.

[github-actions]

✅MegaLinter analysis: Success

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✅ PYTHON	isort	791	3	0	0	2.26s
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✅ REPOSITORY	gitleaks	yes		no	no	15.92s
✅ REPOSITORY	git_diff	yes		no	no	0.12s
✅ REPOSITORY	secretlint	yes		no	no	22.61s
✅ REPOSITORY	syft	yes		no	no	4.06s
✅ REPOSITORY	trivy-sbom	yes		no	no	5.71s
✅ REPOSITORY	trufflehog	yes		no	no	21.67s
✅ YAML	prettier	11	0	0	0	0.67s
✅ YAML	v8r	11		0	0	10.41s
✅ YAML	yamllint	11		0	0	0.45s

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