Fix build on Xcode 16.3 / SDK < 26

[pcuenca]

Building MLXVLM on Xcode 16.3 (SDK < 26) fails under Swift strict concurrency because CIContext is not Sendable. The annotation was added to the macOS 26 SDK. We found this out when bumping mlx-swift-lm to the latest 3.x version in AnyLanguageModel.

Proposed changes

Declare the CIContext var as nonisolated(unsafe). CIContext rendering ops are actually thread-safe. macOS SDK v6 added an @unchecked Sendable annotation that is not present in previous SDKs.

We prepared a test repo that uses GitHub CI to show how builds work with this PR and the official repo, for both Xcode 16.3 and Xcode 26.

Checklist

Put an x in the boxes that apply.

• I have read the CONTRIBUTING document
• I have run pre-commit run --all-files to format my code / installed pre-commit prior to committing changes
• I have added tests that prove my fix is effective or that my feature works (in a separate repo)
• I have updated the necessary documentation (if needed)

[davidkoski]

See also #268 -- I thought this was fixed, but I guess not!

[davidkoski]

Thank you for the fix, just waiting on CI.