Fix Swift 6.0 and 6.1 build failures caused by main-actor isolation crossings.
SceneSize: mutating the@Statesize fromonPreferenceChangefailed on Swift 6.0 across every Apple platform (main actor-isolated property 'size' can not be mutated from a Sendable closure). The mutation now hops throughMainActor.assumeIsolated.WindowSize: the AppKitNSWindow.didResizeNotificationobserver called the main-actor-isolatedemit()from a nonisolated block, failing on Swift 6.1 for macOS. Same fix.SelfSize: the modifier stored a derived(CGSize) -> Voidclosure and captured it insideonPreferenceChange. Function types are not Sendable, so this breaks under the same SDK rule. The modifier now holds theBindingdirectly, which is Sendable, and writes throughMainActor.assumeIsolated.
No public API changes; all three modifiers are private implementation details behind trackSceneSize(), trackWindowSize() and selfSizing(_:).