Reject masked ROI threshold inputs

[FlorianPfaff]

Summary

• detect genuinely masked NumPy values before backend conversion in the ROI threshold compatibility hook
• reject masked similarity entries for both Otsu and minimum-valley threshold estimation
• reject masked nbins scalars while preserving masked-array inputs whose masks are entirely clear
• add focused regressions for direct masks, nested masked entries, masked bin counts, and no-mask compatibility

Bug

Backend asarray conversion drops NumPy mask metadata. As a result, a value intentionally hidden by np.ma.MaskedArray could silently participate in the similarity histogram and change the selected Otsu/minimum threshold. Masked nbins values could likewise be interpreted from their hidden payload.

Rejecting genuine masks before conversion prevents hidden values from influencing ROI association thresholds.

Validation

• Python syntax compilation of the modified compatibility module
• isolated NumPy/SciPy behavior harness covering both threshold functions
• final diff review: two changed files, mergeable against current main
• GitHub Actions test, lint, security, documentation, and dependency workflows are running

Full repository and backend-matrix validation is delegated to GitHub Actions.

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ COPYPASTE	jscpd	yes		no	no	23.89s
✅ JSON	prettier	7	0	0	0	1.07s
✅ JSON	v8r	7		0	0	3.52s
✅ MARKDOWN	markdownlint	68	0	0	0	1.95s
✅ MARKDOWN	markdown-table-formatter	68	0	0	0	0.68s
✅ PYTHON	black	1828	65	0	0	87.87s
✅ PYTHON	isort	1828	99	0	0	2.69s
✅ REPOSITORY	betterleaks	yes		no	no	1.91s
✅ REPOSITORY	checkov	yes		no	no	53.21s
✅ REPOSITORY	gitleaks	yes		no	no	16.69s
✅ REPOSITORY	git_diff	yes		no	no	0.28s
✅ REPOSITORY	secretlint	yes		no	no	80.77s
✅ REPOSITORY	syft	yes		no	no	5.6s
✅ REPOSITORY	trivy-sbom	yes		no	no	5.86s
✅ REPOSITORY	trufflehog	yes		no	no	31.79s
✅ YAML	prettier	11	0	0	0	0.74s
✅ YAML	v8r	11		0	0	12.08s
✅ YAML	yamllint	11		0	0	0.5s

Notices

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