docs: add macOS compiler troubleshooting

[amfred]

Summary

Add macOS contributor troubleshooting for Command Line Tools installs that fail to expose libc++ headers during bundled protobuf builds.

This is an easy fix for a scary looking error, but let me know if you would recommend putting it in a different location than CONTRIBUTING.md. I can't think of a good way to automatically unit test it because the error only shows up on a Mac that's had the Command Line Tools installed for a while, but I did test the fix myself.

Related Issue

N/A

Changes

• Document installing Apple Command Line Tools before local builds.
• Add a specific remediation path for protobuf-src C++ header and C++11 detection failures on macOS.

Testing

• mise run pre-commit passes
• Unit tests added/updated
• E2E tests added/updated (if applicable)

Checklist

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

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[amfred]

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[amfred]

recheck