/// A property wrapper for reactive state owned by a ``View``.
///
/// Mutations fire targeted updates — only widgets that read this state
/// (via ``Binding`` or a tracked closure) are affected. The backing
/// ``StateBox`` is a reference type, so struct copies share the same
/// storage (``nonmutating set`` on a `let` wrapper).
///
/// ```swift
/// struct Counter: View {
///     @State private var count = 0
///     var body: some View {
///         HStack {
///             Button("-") { count -= 1 }
///             Label { "\(count)" }
///             Button("+") { count += 1 }
///         }
///     }
/// }
/// ```
@propertyWrapper @MainActor public struct State<Value> {
    private let box: StateBox<Value>

    public init(wrappedValue: Value) {
        box = StateBox(wrappedValue)
    }

    /// The current value. Reads track dependencies; writes notify.
    public var wrappedValue: Value {
        get { box.get() }
        nonmutating set { box.set(newValue) }
    }

    /// A ``Binding`` to this state, for passing to child views.
    public var projectedValue: Binding<Value> {
        Binding(box)
    }
}