异步场景切换：加载完成事件不能直接激活子场景

public enum TransitionState
{
    Idle,
    Loading,
    ReadyToActivate,
    Failed
}

public readonly record struct LoadTicket(long Generation, string Scene);

public readonly record struct ActivationBatch(
    long Generation,
    IReadOnlyList<string> Scenes);

public sealed class SceneTransitionCoordinator
{
    private readonly HashSet<string> required;
    private readonly HashSet<string> ready = new(StringComparer.Ordinal);
    private long generation;

    public SceneTransitionCoordinator(IEnumerable<string> requiredScenes)
    {
        required = new HashSet<string>(requiredScenes, StringComparer.Ordinal);
        if (required.Count == 0 || required.Any(string.IsNullOrWhiteSpace))
        {
            throw new ArgumentException(
                "At least one named scene is required.",
                nameof(requiredScenes));
        }
    }

    public TransitionState State { get; private set; } = TransitionState.Idle;
    public string? Failure { get; private set; }

    public IReadOnlyList<LoadTicket> Begin()
    {
        generation++;
        ready.Clear();
        Failure = null;
        State = TransitionState.Loading;
        return required.Order()
            .Select(scene => new LoadTicket(generation, scene))
            .ToArray();
    }

    public bool ReportReady(LoadTicket ticket)
    {
        if (!Accepts(ticket)) return false;

        ready.Add(ticket.Scene);
        if (ready.SetEquals(required))
        {
            State = TransitionState.ReadyToActivate;
        }
        return true;
    }

    public bool ReportFailed(LoadTicket ticket, string reason)
    {
        if (!Accepts(ticket) || string.IsNullOrWhiteSpace(reason))
        {
            return false;
        }

        Failure = reason;
        State = TransitionState.Failed;
        return true;
    }

    public ActivationBatch Commit()
    {
        if (State != TransitionState.ReadyToActivate)
        {
            throw new InvalidOperationException(
                "Every required scene must be ready before activation.");
        }

        State = TransitionState.Idle;
        return new ActivationBatch(generation, required.Order().ToArray());
    }

    private bool Accepts(LoadTicket ticket)
    {
        return State == TransitionState.Loading
            && ticket.Generation == generation
            && required.Contains(ticket.Scene);
    }
}

$ dotnet build -c Release --nologo -p:NuGetAudit=false
Build succeeded.
    0 Warning(s)
    0 Error(s)
Time Elapsed 00:00:06.26

$ dotnet run -c Release --no-build --no-restore
exitCode=0
passed=12 failed=0 skipped=0 elapsedMs=70.7343
input=Terrain,Lighting,Gameplay
states=Loading,Loading,ReadyToActivate
activationGeneration=1
activated=Gameplay,Lighting,Terrain