fix(ci): let setup-julia pick the runner arch (macOS legs)

[hyperpolymath]

macos-latest is now an arm64 (Apple Silicon) image. The matrix forced arch: x64, so setup-julia refused before any test ran ("x64 arch has been requested on a macOS runner that has an arm64 architecture"). Dropping the arch axis lets setup-julia use the native default on every OS: aarch64 on macOS, x64 on ubuntu/windows.

Claude-Session: https://claude.ai/code/session_012kgrMQRhSmZMBbF9Ui1zBw

Summary

Closes #

Type of change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality)
• 💥 Breaking change (would change existing behaviour)
• 🕳️ Soundness fix (fixes a checker/proof false-negative)
• 📖 Documentation
• 🧹 Refactor / tech debt (behaviour-preserving)
• ⚡ Performance
• 🔧 Build / CI / tooling

How has this been verified?

Checklist

• My commits are signed (git commit -S).
• I ran the project's own checks/tests locally and they pass.
• New files carry the correct SPDX-License-Identifier (code/config MPL-2.0,
  prose CC-BY-SA-4.0); I did not relicense existing files.
• Docs are updated, and no public claim now overstates what the code does.
• I have not introduced a soundness hole (or I have flagged where I might have).

Notes for reviewers

[coderabbitai]

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📥 Commits

Reviewing files that changed from the base of the PR and between c0ba04e and 60cdf5b.

📒 Files selected for processing (1)

• .github/workflows/CI.yml

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