Validate Pareto directions before empty result

[FlorianPfaff]

Summary

• Validate pareto_front_indices objective directions before taking the empty-table fast path.
• Add a regression test showing invalid directions are rejected even when the input table has no rows.

Bug

Before this change, pareto_front_indices() returned [] for an empty table before validating directions, so invalid directions such as "sideways" were silently accepted. This was inconsistent with is_pareto_front() and with non-empty Pareto calls.

Testing

• Added focused regression test in tests/evaluation/test_pareto_empty_validation.py.
• Not run locally; this environment cannot clone GitHub or install the repository dependencies, so CI should run the regression test on the PR.

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ COPYPASTE	jscpd	yes		no	no	56.51s
✅ JSON	prettier	7	0	0	0	1.2s
✅ JSON	v8r	7		0	0	5.27s
✅ MARKDOWN	markdownlint	68	0	0	0	1.74s
✅ MARKDOWN	markdown-table-formatter	68	0	0	0	0.82s
✅ PYTHON	black	891	10	0	0	56.26s
✅ PYTHON	isort	891	11	0	0	3.75s
✅ REPOSITORY	checkov	yes		no	no	48.46s
✅ REPOSITORY	gitleaks	yes		no	no	9.59s
✅ REPOSITORY	git_diff	yes		no	no	0.08s
✅ REPOSITORY	secretlint	yes		no	no	26.14s
✅ REPOSITORY	syft	yes		no	no	5.21s
✅ REPOSITORY	trivy-sbom	yes		no	no	7.96s
✅ REPOSITORY	trufflehog	yes		no	no	25.1s
✅ YAML	prettier	11	0	0	0	0.73s
✅ YAML	v8r	11		0	0	12.26s
✅ YAML	yamllint	11		0	0	0.51s

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