Annotate UIKit-bound controllers with @MainActor for Swift 6

[Xopoko]

Summary

• mark UIKit-facing helper extensions and controllers as @MainActor
• fixes Swift 6 SWIFT_STRICT_CONCURRENCY=complete builds by allowing constraint updates and animator callbacks on the main actor

Details

Building with Swift 6.2 and SWIFT_SUPPRESS_WARNINGS=NO currently fails due to main-actor isolation violations in:

• UIViewController+Children.swift (helper methods used by container)
• HeightContraintOverlayTranslationController.swift (mutates NSLayoutConstraint)
• InterruptibleAnimatorOverlayContainerTransitionCoordinator.swift (animator callbacks)

Adding @MainActor to these types brings them in line with UIKit’s main-thread requirement and resolves the concurrency errors. Behavior is unchanged; only actor isolation is specified explicitly.

Tested: builds cleanly on Swift 6.2 simulator with strict concurrency.

[gaetanzanella]

Hey @Xopoko thank you for the MR!
Could you add this @mainactor everywhere? 🙏 every component should have it

[Xopoko]

Thanks — agreed. I rechecked current head 2cf595be396477f63aa66169847b20120567b3f2; it only annotates three UIKit-bound types, so this remains incomplete. I’m treating the request as actionable and will follow up here with the successor SHA and build/test evidence once the full component set is updated.