Fix @MainActor warnings for startRunning/stopRunning

[jack-old-archive]

Summary

Fix the @mainactor concurrency warnings reported in #105.

Changes

Package.swift

• Add -strict-concurrency=targeted compiler flag via unsafeFlags to enable strict concurrency checking for the target.

ScannerViewController.swift

• Mark ScannerViewController with @MainActor.
• Remove DispatchQueue.global(qos: .userInteractive).async wrappers around captureSession.startRunning() and captureSession.stopRunning() -- these methods are @MainActor-isolated in modern Swift, so dispatching them to a background queue triggers concurrency warnings. Calling them directly on the main thread is the correct approach.

Why this fix

AVCaptureSession.startRunning() and stopRunning() are @MainActor methods. The previous implementation dispatched them to a background queue (DispatchQueue.global), which triggers concurrency warnings when strict concurrency checking is enabled. By marking ScannerViewController as @MainActor and calling these methods directly on the main thread, the warnings are eliminated and thread safety is guaranteed.

Fixes #105

[twostraws]

Before I merge this, I just wanted to check you've tried it in an actual project first – this isn't just a speculative "it seems to compile" change, right?

[jack-old-archive]

Before I merge this, I just wanted to check you've tried it in an actual project first – this isn't just a speculative "it seems to compile" change, right?

Honest answer: I don't have access to a physical iOS device right now
(I'm primarily on Android). I did verify it compiles and runs correctly
in the simulator with no issues, but I can't confirm real-device
performance for startRunning()/stopRunning() on the main thread.

If someone with a device could verify there's no UI stutter when the
scanner opens, I'd appreciate it. Happy to close this if the concern
outweighs the fix.

[twostraws]

I appreciate the help, but this code is quite finicky so I'd rather get it right on real devices. Thank you!