ci: Add cargo-deny + cargo-audit + cargo-mutants nightly workflow

[nanaf6203-bit]

Restore the nightly cargo-mutants workflow (#547) and add cargo-deny and cargo-audit to the same workflow. Surface results in AUDIT_LOG.md.

[Israel-light]

@Israel-light has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

I’m interested in this issue because it strengthens the repository’s supply-chain and code-quality guarantees by adding automated dependency auditing and mutation testing. Combining cargo-deny, cargo-audit, and cargo-mutants in a nightly workflow provides early detection of vulnerabilities, licensing issues, and weak test coverage.

I’ve worked with Rust CI pipelines and GitHub Actions workflows, including integrating cargo-audit, dependency policy checks, and test coverage/mutation tooling. I’m comfortable setting up reproducible nightly jobs and ensuring outputs are properly surfaced for review.

For this task, I would:

restore the existing nightly cargo-mutants workflow,
integrate cargo-deny and cargo-audit into the same CI job,
ensure all outputs are captured and written to AUDIT_LOG.md,
and structure the workflow so failures are clearly reported but non-blocking if required.

I would also ensure the workflow runs efficiently and produces consistent, readable audit logs for maintainers.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Israel-light to this issue.

[anumukul]

@anumukul has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hi, I’d like to take this issue and can deliver a fix within 24 hours.
I’ve worked on similar projects before and have relevant experience, so I should be able to handle this efficiently.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @anumukul to this issue.

[warisu]

@warisu has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hello, I can handle this, my ETA is 12hrs

ℹ️ Repo Maintainers: To accept this application, review their application or assign @warisu to this issue.

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Congratulations, @warisu! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

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This issue has been completed by @warisu as part of the Stellar Wave Program's 6th Wave 🥳

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