Avoid cancellation in Merwe sigma-point scaling

[FlorianPfaff]

Summary

• compute the Merwe sigma-point scale directly as alpha**2 * (n + kappa)
• stop reconstructing that small positive quantity through n + (scale - n)
• validate underflowed or overflowed scale values with a clear ValueError
• add a regression for alpha=1e-9

Bug

MerweScaledSigmaPoints first computed

lambda_ = alpha**2 * (n + kappa) - n
scale = n + lambda_

For valid small positive alpha, the subtraction can round lambda_ to exactly -n. Adding n back then produces 0.0, even though the direct scale remains finite and positive. With n=1, kappa=0, and alpha=1e-9, the old implementation computes lambda_ == -1.0 and scale == 0.0, causing division by zero during weight construction.

Fix

Use the mathematically equivalent direct expression for both weight construction and covariance scaling. A shared helper also rejects genuine scale underflow or overflow explicitly.

Validation

• isolated reproduction confirms the old reconstruction yields 0.0 while the direct expression yields 1e-18
• isolated execution verifies finite weights and sigma points [0, +1e-9, -1e-9]
• existing two-dimensional Merwe moment reconstruction still recovers the original mean and covariance
• modified source and regression modules pass python -m py_compile
• branch is two commits ahead of main and zero behind

[github-actions]

✅MegaLinter analysis: Success

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✅ REPOSITORY	git_diff	yes		no	no	0.48s
✅ REPOSITORY	secretlint	yes		no	no	63.15s
✅ REPOSITORY	syft	yes		no	no	5.89s
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✅ REPOSITORY	trufflehog	yes		no	no	32.22s
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✅ YAML	v8r	11		0	0	10.98s
✅ YAML	yamllint	11		0	0	0.64s

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