Add generic pairwise covariance association features

[FlorianPfaff]

Summary

• Add domain-neutral pairwise Gaussian/covariance association feature utilities.
• Export pairwise_mahalanobis_distances and pairwise_covariance_shape_components from pyrecest.utils.
• Add unit tests covering summed-covariance Mahalanobis distances, regularization, empty stacks, covariance shape/log-det features, and invalid inputs.
• Document the new utilities in the API overview.

Testing

• Not run in GitHub connector environment.
• Previously validated the generated test file in a local NumPy-backed stub environment before pushing the branch.

Notes

The branch is based on a recent main, but main advanced while this was being prepared. The diff is limited to four intended files and should be straightforward to update if GitHub marks the branch stale.

[github-actions]

Test Results

    12 files  +    12      12 suites  +12   2h 28m 15s ⏱️ + 2h 28m 15s
 1 497 tests + 1 497   1 497 ✅ + 1 497      0 💤 ±    0  0 ❌ ±0 
18 424 runs  +18 424  14 712 ✅ +14 712  3 712 💤 +3 712  0 ❌ ±0 

Results for commit 63299a6. ± Comparison against base commit 876d5cc.

♻️ This comment has been updated with latest results.

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ COPYPASTE	jscpd	yes		no	no	17.12s
✅ JSON	prettier	2	0	0	0	0.48s
✅ JSON	v8r	2		0	0	3.27s
✅ MARKDOWN	markdownlint	28	0	0	0	1.35s
✅ MARKDOWN	markdown-table-formatter	28	0	0	0	0.31s
✅ PYTHON	bandit	507		0	0	8.99s
✅ PYTHON	black	507	5	0	0	14.32s
✅ PYTHON	flake8	507		0	0	4.89s
✅ PYTHON	isort	507	7	0	0	0.98s
✅ PYTHON	mypy	507		0	0	7.67s
✅ PYTHON	pylint	507		0	0	127.5s
✅ PYTHON	ruff	507	7	0	0	0.1s
✅ REPOSITORY	checkov	yes		no	no	28.11s
✅ REPOSITORY	gitleaks	yes		no	no	14.33s
✅ REPOSITORY	git_diff	yes		no	no	0.06s
✅ REPOSITORY	secretlint	yes		no	no	12.9s
✅ REPOSITORY	syft	yes		no	no	3.11s
✅ REPOSITORY	trivy-sbom	yes		no	no	3.63s
✅ REPOSITORY	trufflehog	yes		no	no	23.07s
✅ YAML	prettier	5	0	0	0	0.56s
✅ YAML	v8r	5		0	0	6.8s
✅ YAML	yamllint	5		0	0	0.48s

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