Media and utilities Swift 6

[stevenzeck]

• Removed Atomic.swift and the @Atomic property wrapper.
• Isolated Navigator, Configurable, AudioSessionUser, PublicationSpeechSynthesizer, and NowPlayingInfo to @MainActor.
• Implemented @unchecked Sendable wrappers (TimeObserverToken and NotificationObserverToken) for AVPlayer KVO and NotificationCenter.
• Updated Cancellable protocol, CancellableObject, and MediatorCancellable to conform to Sendable, using Mutex to secure callbacks.
• Created a dedicated Poller class for condition-polling, requiring @Sendable and @MainActor isolation for all callbacks.
• Isolated UserPreferencesViewModel and TTSViewModel to @MainActor and utilized MainActor.assumeIsolated within Combine callbacks to resolve compilation errors in the TestApp.

Public API Breaking Changes

• Navigator, Configurable, AudioSessionUser, PublicationSpeechSynthesizer, and NowPlayingInfo are now @MainActor isolated.
• Atomic class and @Atomic property wrapper removed from ReadiumShared.
• Cancellable, CancellableObject, and MediatorCancellable now conform to Sendable.
• AudioSession.end(for:) parameter type changed from AudioSessionUser to ObjectIdentifier.
• execute(when:pollingInterval:on::) parameters and return type updated to require @Sendable @MainActor.
• CancellableObject.init(onCancel:) closure now requires @Sendable.
• throttle(duration:on::) closure and return type now require @Sendable.

Relates to #758.

[Copilot]

Pull request overview

This PR advances Swift 6 / strict concurrency compliance across the media/toolkit utilities and navigator layers by tightening actor isolation (@MainActor) and updating utility primitives and callbacks to be Sendable-safe.

Changes:

• Isolates key navigator/media APIs and related view models to @MainActor.
• Reworks control-flow and cancellation utilities to require @Sendable callbacks and uses Mutex for thread-safe state.
• Introduces a polling utility (execute(when:...) via a new Poller) and adds observer-token wrappers for AVPlayer/NotificationCenter observer lifetime management.

Reviewed changes

Copilot reviewed 13 out of 13 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
TestApp/Sources/Reader/Common/TTS/TTSViewModel.swift	Marks the view model as @MainActor to satisfy strict concurrency.
TestApp/Sources/Reader/Common/Preferences/UserPreferences.swift	Marks preferences view model @MainActor and adjusts Combine callbacks for actor isolation.
Sources/Shared/Toolkit/Poller.swift	Adds a dedicated polling implementation and updates execute(when:...) to require @Sendable @MainActor callbacks.
Sources/Shared/Toolkit/Media/NowPlayingInfo.swift	Isolates Now Playing manager to @MainActor and adapts throttled updates.
Sources/Shared/Toolkit/Media/AudioSession.swift	Moves audio session user protocol + session management onto @MainActor and adjusts APIs for Swift 6.
Sources/Shared/Toolkit/ControlFlow.swift	Updates throttle to be @Sendable-friendly and thread-safe via Mutex.
Sources/Shared/Toolkit/Cancellable.swift	Makes cancellation primitives Sendable and guards state/callbacks with Mutex.
Sources/Shared/Toolkit/Atomic.swift	Removes the legacy Atomic wrapper from ReadiumShared.
Sources/Navigator/VisualNavigator.swift	Marks VisualNavigator default implementations as @MainActor.
Sources/Navigator/TTS/PublicationSpeechSynthesizer.swift	Isolates synthesizer to @MainActor and updates delegate interactions and teardown.
Sources/Navigator/Preferences/Configurable.swift	Marks Configurable as @MainActor to align UI-facing configuration flows with isolation.
Sources/Navigator/Navigator.swift	Marks Navigator (and convenience extension) as @MainActor.
Sources/Navigator/Audiobook/AudioNavigator.swift	Isolates AudioNavigator to @MainActor, introduces observer token wrappers, and updates callback/sendability to satisfy Swift 6.

Comments suppressed due to low confidence (1)

Sources/Navigator/Audiobook/AudioNavigator.swift:375

• completion is declared @MainActor, but it’s called from a DispatchQueue.main.async closure which is not guaranteed to be on the MainActor executor. This will typically trigger a Swift 6 actor-isolation error unless you explicitly assert/hop onto the MainActor before calling completion.

            DispatchQueue.main.async {
                completion(info)
            }

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