Stabilize Merwe sigma-point scaling

[FlorianPfaff]

Bug

MerweScaledSigmaPoints computed the positive sigma-point scale as n + lambda after first evaluating lambda = alpha**2 * (n + kappa) - n. For sufficiently small but representable positive alpha, subtracting n rounds lambda to exactly -n; adding n back then yields zero. Construction consequently raises ZeroDivisionError, and sigma-point generation would use a zero Cholesky scale.

For example, n=2, kappa=0, and alpha=1e-10 have the valid direct scale 2e-20, but the previous cancellation-prone reconstruction produces 0.0.

Fix

• compute and retain alpha * alpha * (n + kappa) directly
• use that retained value for both weights and covariance scaling
• reject only genuine scale underflow or overflow with a clear ValueError
• add focused regressions for the small-alpha cancellation case and for non-representable scales

Validation

• reproduced the previous ZeroDivisionError analytically for alpha=1e-10
• syntax-compiled the modified implementation and regression
• ran the exact regression through an isolated NumPy-backed PyRecEst harness: 2 passed
• branch is 2 commits ahead of main, 0 behind, with 10 implementation-line changes and one focused test file

Full repository CI should run on this pull request.

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
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✅ PYTHON	black	1543	197	0	0	95.16s
✅ PYTHON	isort	1543	367	0	0	2.61s
✅ REPOSITORY	betterleaks	yes		no	no	2.65s
✅ REPOSITORY	checkov	yes		no	no	56.06s
✅ REPOSITORY	gitleaks	yes		no	no	15.03s
✅ REPOSITORY	git_diff	yes		no	no	0.31s
✅ REPOSITORY	secretlint	yes		no	no	74.99s
✅ REPOSITORY	syft	yes		no	no	5.17s
✅ REPOSITORY	trivy-sbom	yes		no	no	6.68s
✅ REPOSITORY	trufflehog	yes		no	no	32.99s
✅ YAML	prettier	11	0	0	0	0.84s
✅ YAML	v8r	11		0	0	12.04s
✅ YAML	yamllint	11		0	0	0.81s

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