Update rustc crate rkyv to 0.8.18

[rperier]

See
https://rustsec.org/advisories/RUSTSEC-2026-0233
https://rustsec.org/advisories/RUSTSEC-2026-0234
https://rustsec.org/advisories/RUSTSEC-2026-0235

• I did not use an LLM to create a change in this PR.
• I used an LLM to create a change in this PR, and I have explained below how it was used.

[rustbot]

These commits modify the Cargo.lock file. Unintentional changes to Cargo.lock can be introduced when switching branches and rebasing PRs.

If this was unintentional then you should revert the changes before this PR is merged.
Otherwise, you can ignore this comment.

[rustbot]

r? @Mark-Simulacrum

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• Owners of files modified in this PR: @Mark-Simulacrum