book: document platform security scanning

[ColumbusLabs]

Summary

• explain why platform security scanning can slow mutation-test workloads
• point Windows users to Dev Drive performance mode or narrowly scoped directory exclusions
• point macOS users to the Developer Tools allowance for XProtect and Gatekeeper
• explicitly avoid recommending that users disable system protections globally

Closes #189.

Validation

• typos book/src/performance.md
• verified both linked nextest guidance pages return HTTP 200 and support the described mitigations
• git diff --check

mdbook and mdbook-linkcheck were not installed locally, so the rendered book build was not rerun.

[Copilot]

Pull request overview

Adds documentation to the performance guide explaining how platform security scanning can slow down cargo-mutants runs, and points readers to platform-specific mitigations that avoid disabling protections globally.

Changes:

• Documented why antivirus / platform security scanning can noticeably slow mutation-test workloads.
• Added Windows guidance (Dev Drive performance mode or scoped Windows Security exclusions) and macOS guidance (Developer Tools allowance for the terminal via nextest docs).

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