Add cargo_mutants_version field to outcomes.json

[Copilot]

• Add cargo_mutants_version field to LabOutcome struct in src/outcome.rs
• Set version directly from crate::VERSION in LabOutcome::new() (no parameter needed)
• Strip cargo_mutants_version in test utility to avoid brittle version-specific assertions
• Fold version check into existing small_well_tested_tree_is_clean test
• Run tests to verify the change works correctly
• Update documentation in NEWS.md and book/src/mutants-out.md

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add a cargo_mutants_version field into LabOutcome written to outcomes.json.</issue_title>
<issue_description></issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes #561

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