docs: verify the source build on an Intel Mac

[roooooly]

Goal

The current automated and local checks cover Apple Silicon. We need a privacy-safe Intel Mac source-build report before stating broader architecture support.

Acceptance criteria

• Record the macOS and Xcode versions and the tested commit.
• Run xcodegen generate, the privacy audit, core tests, and the app test command.
• Use only synthetic projects and session fixtures.
• Report the exact first failure if the full suite does not pass; do not upload an .xcresult bundle.
• Update the README or troubleshooting notes with the verified boundary.

A successful build report is compatibility evidence for that configuration, not a universal performance claim.