fix(mxc): resolve Windows ETW Clippy lints

[pimlock]

Summary

Resolve the Clippy warnings introduced by #3015 that break the Windows arm64 lint job. The changes follow Clippy's idiomatic rewrites without changing ETW capture, attribution, or OCSF behavior.

Related Issue

No issue required: this is a localized mechanical follow-up to #3015 and Actions job 103116524277.

Changes

• Simplify control flow and Option/Result handling flagged by Clippy.
• Use the mutable pointer and explicit unsigned-cast APIs for ETW interop.
• Replace allocation-heavy GUID formatting and clean up documentation formatting.
• Update test fixtures to use direct struct initialization and checked Instant subtraction.

Testing

• mise run --skip-tools windows:lint:arm64
• mise run pre-commit

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)

Automated follow-up for #3015 and Actions job 103116524277

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[pimlock]

/ok to test 8dd3e60