Gate private macOS blur APIs behind an opt-in feature

[zcourts]

Summary

• add a default-disabled private-apple-apis feature
• compile the private CGS blur declarations and calls only when explicitly enabled
• keep the public/default macOS build free of _CGSMainConnectionID and _CGSSetWindowBackgroundBlurRadius
• document the App Store compatibility impact

Validation

• Linux checks with the feature disabled and enabled
• macOS ARM64 and x86_64 checks with the feature disabled and enabled
• nm confirms both CGS symbols are absent from default builds and present only in explicit opt-in builds

This is a targeted backport of the upstream opt-in gating needed by the Fission 0.30.13 fork.