[codex] Fix KernelSME empty gated update

[FlorianPfaff]

Summary

• Treat an empty Kernel SME measurement set after gating as a no-op posterior update.
• Preserve state and covariance when all measurements are rejected by gating.
• Add a regression test covering the all-measurements-rejected path.

Root Cause

KernelSMEFilter.update_linear filtered measurements during gating, then unconditionally called gen_test_points. If gating rejected every measurement, gen_test_points attempted vstack([]) and failed.

Validation

• Not run locally; this Codex workspace is read-only.
• Added test_gating_rejecting_all_measurements_keeps_prior to cover the previously failing path.

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ COPYPASTE	jscpd	yes		no	no	18.36s
✅ JSON	prettier	2	0	0	0	0.42s
✅ JSON	v8r	2		0	0	2.9s
✅ MARKDOWN	markdownlint	2	0	0	0	0.7s
✅ MARKDOWN	markdown-table-formatter	2	0	0	0	0.21s
✅ PYTHON	bandit	382		0	0	5.66s
✅ PYTHON	black	382	1	0	0	8.65s
✅ PYTHON	flake8	382		0	0	3.22s
✅ PYTHON	isort	382	1	0	0	0.73s
✅ PYTHON	mypy	382		0	0	5.07s
✅ PYTHON	pylint	382		0	0	117.41s
✅ PYTHON	ruff	382	1	0	0	0.07s
✅ REPOSITORY	checkov	yes		no	no	22.33s
✅ REPOSITORY	gitleaks	yes		no	no	10.31s
✅ REPOSITORY	git_diff	yes		no	no	0.06s
✅ REPOSITORY	secretlint	yes		no	no	7.15s
✅ REPOSITORY	syft	yes		no	no	3.54s
✅ REPOSITORY	trivy-sbom	yes		no	no	2.26s
✅ REPOSITORY	trufflehog	yes		no	no	20.34s
✅ YAML	prettier	4	0	0	0	0.48s
✅ YAML	v8r	4		0	0	4.71s
✅ YAML	yamllint	4		0	0	0.42s

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