Stabilize discrete-state prior normalization

[FlorianPfaff]

Summary

• normalize finite discrete-state priors after scaling by their largest entry
• prevent summation overflow from turning valid priors into all-zero vectors
• add public-API regressions for HMM and IMM state/mode priors near the float64 limit

Root cause

_validated_probability_vector() summed caller-provided probabilities at their original scale. A vector such as [np.finfo(float).max, np.finfo(float).max / 2] contains only finite, non-negative entries, but its sum overflows to infinity. Dividing by that total produces an all-zero prior, and the subsequent filtering recursion fails because no prior mass remains.

Fix

Scale the validated and masked vector by its maximum positive entry before summing. The scaled sum is finite and at least one, while all probability ratios and zero support are preserved.

Validation

Added focused regressions through discrete_forward_backward() and imm_forward_backward() covering state and mode priors at the maximum finite float64 scale.

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ COPYPASTE	jscpd	yes		no	no	24.84s
✅ JSON	prettier	7	0	0	0	1.21s
✅ JSON	v8r	7		0	0	4.65s
✅ MARKDOWN	markdownlint	68	0	0	0	1.94s
✅ MARKDOWN	markdown-table-formatter	68	0	0	0	0.54s
✅ PYTHON	black	1542	197	0	0	98.6s
✅ PYTHON	isort	1542	366	0	0	2.8s
✅ REPOSITORY	betterleaks	yes		no	no	2.62s
✅ REPOSITORY	checkov	yes		no	no	56.6s
✅ REPOSITORY	gitleaks	yes		no	no	15.45s
✅ REPOSITORY	git_diff	yes		no	no	0.43s
✅ REPOSITORY	secretlint	yes		no	no	63.71s
✅ REPOSITORY	syft	yes		no	no	6.59s
✅ REPOSITORY	trivy-sbom	yes		no	no	7.16s
✅ REPOSITORY	trufflehog	yes		no	no	32.83s
✅ YAML	prettier	11	0	0	0	0.77s
✅ YAML	v8r	11		0	0	11.44s
✅ YAML	yamllint	11		0	0	0.66s

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