Add sparse second-order grid evidence

[FlorianPfaff]

Summary

• add a reusable sparse second-order grid evidence utility
• expose the utility through lazy filter exports
• document the helper and add focused coverage for evidence and smoothing behavior

Verification

• Not run, per request: tests
• python -m py_compile src\pyrecest\filters\sparse_second_order_grid.py tests\test_sparse_second_order_grid.py
• git diff --check

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ COPYPASTE	jscpd	yes		no	no	44.0s
✅ JSON	prettier	7	0	0	0	1.0s
✅ JSON	v8r	7		0	0	4.3s
✅ MARKDOWN	markdownlint	67	0	0	0	1.53s
✅ MARKDOWN	markdown-table-formatter	67	0	0	0	0.47s
✅ PYTHON	black	788	2	0	0	41.22s
✅ PYTHON	isort	788	2	0	0	2.06s
✅ REPOSITORY	checkov	yes		no	no	42.91s
✅ REPOSITORY	gitleaks	yes		no	no	13.22s
✅ REPOSITORY	git_diff	yes		no	no	0.14s
✅ REPOSITORY	secretlint	yes		no	no	24.3s
✅ REPOSITORY	syft	yes		no	no	4.83s
✅ REPOSITORY	trivy-sbom	yes		no	no	6.32s
✅ REPOSITORY	trufflehog	yes		no	no	22.44s
✅ YAML	prettier	11	0	0	0	0.72s
✅ YAML	v8r	11		0	0	10.41s
✅ YAML	yamllint	11		0	0	0.45s

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