Stabilize IMM transition-matrix normalization

[FlorianPfaff]

Summary

• normalize nonnegative IMM transition rows after scaling by their largest entry
• avoid overflow when individually finite row weights have a non-finite direct sum
• add a focused regression using maximum finite floating-point weights

Bug

InteractingMultipleModelFilter._prepare_transition_matrix() normalized non-unit rows with a direct row sum:

transition_matrix / transition_matrix.sum(axis=1)[:, None]

A row can contain valid, finite, nonnegative weights whose sum overflows. For example, [finfo.max, finfo.max / 2] has the well-defined ratio 2:1, but its floating-point sum is inf. The old normalization therefore produced [0, 0], leaving a transition row with no probability mass.

Fix

Use each row's maximum entry as a scale before summing. The scaled entries lie in [0, 1], retain the original ratios, and can be normalized without overflowing. Already normalized rows remain unchanged and do not emit the existing renormalization warning.

Validation

• isolated numerical reproduction: old result [0, 0]; fixed result [2/3, 1/3]
• regression runs under np.errstate(over="raise", invalid="raise", divide="raise")
• branch comparison against main: two commits ahead, zero behind; one source file and one focused test file changed

The full repository test matrix is delegated to GitHub Actions.

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ COPYPASTE	jscpd	yes		no	no	23.94s
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✅ MARKDOWN	markdownlint	68	0	0	0	2.19s
✅ MARKDOWN	markdown-table-formatter	68	0	0	0	0.59s
✅ PYTHON	black	1490	183	0	0	93.82s
✅ PYTHON	isort	1490	328	0	0	2.62s
✅ REPOSITORY	betterleaks	yes		no	no	2.47s
✅ REPOSITORY	checkov	yes		no	no	54.09s
✅ REPOSITORY	gitleaks	yes		no	no	15.25s
✅ REPOSITORY	git_diff	yes		no	no	0.43s
✅ REPOSITORY	secretlint	yes		no	no	59.5s
✅ REPOSITORY	syft	yes		no	no	5.79s
✅ REPOSITORY	trivy-sbom	yes		no	no	7.09s
✅ REPOSITORY	trufflehog	yes		no	no	30.95s
✅ YAML	prettier	11	0	0	0	0.75s
✅ YAML	v8r	11		0	0	11.75s
✅ YAML	yamllint	11		0	0	0.67s

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