Adopt Swift 6 + concurrency + remove background diffing

[jessesquires]

Closes #157.

• Adopt Swift 6
• Support Concurrency
• Remove background diffing

Maintaining CollectionViewDriverOptions.diffOnBackgroundQueue is too large of an effort for very little benefit. It was always questionable, and against guidance from the UIKit team.

See this blog post for more:

https://www.jessesquires.com/blog/2024/12/19/diffable-data-source-main-actor-inconsistency

From the UIKit team:

We have seen a number of issues stemming from usage of diffable data source on background queues/threads, and the performance benefits of doing this are generally minimal due to the fact that only the diffing of identifiers in the old & new snapshots happens on the background queue/thread; the work to set up and execute the Ul updates and animations for cells always happens on the main thread. Therefore, we made the decision to restrict diffable data source to the main actor when using Swift Concurrency, as this ensures correctness in all cases and is nearly always the best approach anyways. If you were previously applying snapshots from a background queue, we recommend you update your implementation to do so on the main queue instead.

[Copilot]

Pull request overview

This PR modernizes the ReactiveCollectionsKit library by adopting Swift 6 and removing the background diffing feature. The change simplifies the codebase by eliminating the CollectionViewDriverOptions.diffOnBackgroundQueue option and its associated implementation, which was always questionable and contrary to UIKit team guidance. All diffing now occurs on the main thread with proper @MainActor isolation, aligning with Swift 6's strict concurrency model.

Key changes:

• Updated to Swift 6.0 language mode across Package.swift, Xcode project, and documentation
• Removed CollectionViewDriverOptions.diffOnBackgroundQueue property and related background queue infrastructure
• Simplified DiffableDataSource by removing nonisolated annotations and MainActor.assumeIsolated wrappers, relying on @MainActor class isolation instead

Reviewed changes

Copilot reviewed 13 out of 13 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
Package.swift	Updated Swift language mode from .v5 to .v6, removed experimental StrictConcurrency settings
ReactiveCollectionsKit.xcodeproj/project.pbxproj	Updated SWIFT_VERSION from 5.0 to 6.0 for both Debug and Release configurations
README.md	Updated Swift version requirement from 5.10+ to 6.0+
Sources/CollectionViewDriverOptions.swift	Removed diffOnBackgroundQueue property and its associated parameter from initializer
Sources/CollectionViewDriver.swift	Removed diffOnBackgroundQueue parameter when initializing DiffableDataSource
Sources/DiffableDataSource.swift	Removed background queue infrastructure, _diffingQueue property, and conditional background/main queue logic; simplified methods by removing nonisolated and MainActor.assumeIsolated calls
Sources/DebugDescriptions.swift	Removed diffOnBackgroundQueue field from debug descriptions
Tests/Utils/CollectionViewDriverOptions+Extensions.swift	Updated test helper to remove diffOnBackgroundQueue parameter
Tests/TestCollectionViewDriverOptions.swift	Removed tests for diffOnBackgroundQueue property and updated debug description assertions
Tests/TestDebugDescriptionDriver.swift	Updated expected debug output to remove diffOnBackgroundQueue field
Tests/TestCollectionViewDriver.swift	Updated test to use default options instead of explicitly setting diffOnBackgroundQueue: true
Example/Sources/List/ListViewController.swift	Removed diffOnBackgroundQueue option from driver initialization
CHANGELOG.md	Added 0.2.0 release notes documenting the breaking change

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