资源预载完成不等于场景可切换：用原子激活屏障保住旧世界

public readonly record struct SceneSnapshot(
    string Name,
    long Generation,
    IReadOnlySet<int> ResidentResources);

public readonly record struct SceneTicket(long Generation, string Name);

public enum ActivationResult
{
    Activated,
    MissingResource,
    StaleTicket,
    ActivationFailed
}

public sealed class SceneActivationBarrier(SceneSnapshot initial)
{
    private SceneSnapshot? _candidate;

    public SceneSnapshot Active { get; private set; } = initial;

    public SceneTicket Stage(
        long generation,
        string name,
        IEnumerable<int> loadedResources)
    {
        if (generation <= Active.Generation)
            throw new ArgumentOutOfRangeException(nameof(generation));

        _candidate = new SceneSnapshot(
            name, generation, loadedResources.ToHashSet());
        return new SceneTicket(generation, name);
    }

    public ActivationResult TryActivate(
        SceneTicket ticket,
        IReadOnlySet<int> requiredResources,
        Action<SceneSnapshot> activate)
    {
        if (_candidate is not { } candidate ||
            candidate.Generation != ticket.Generation ||
            candidate.Name != ticket.Name)
            return ActivationResult.StaleTicket;

        if (!requiredResources.IsSubsetOf(candidate.ResidentResources))
            return ActivationResult.MissingResource;

        try
        {
            activate(candidate);
        }
        catch
        {
            return ActivationResult.ActivationFailed;
        }

        Active = candidate;
        _candidate = null;
        return ActivationResult.Activated;
    }
}

var barrier = new SceneActivationBarrier(
    new("Harbor", 40, new HashSet<int> { 1, 2 }));
var ticket = barrier.Stage(41, "Dungeon", [101, 102]);

var result = barrier.TryActivate(
    ticket,
    new HashSet<int> { 101, 102, 103 },
    _ => { });

Equal(ActivationResult.MissingResource, result,
    "missing resource rejected");
Equal("Harbor", barrier.Active.Name,
    "missing resource preserves active scene");

$ dotnet build $WORK_DIR/probe/SceneActivationProbe.csproj -c Release --no-restore
Build succeeded.
    0 Warning(s)
    0 Error(s)
Time Elapsed 00:00:06.11

$ dotnet run --project $WORK_DIR/probe/SceneActivationProbe.csproj -c Release --no-build --no-restore
missing: ticket=41 loaded=101,102 required=101,102,103 result=MissingResource active=Harbor@40
success: ticket=41 loaded=101,102,103 result=Activated active=Dungeon@41
stale: ticket=41 latest=42 result=StaleTicket active=Harbor@40
failure: ticket=41 result=ActivationFailed active=Harbor@40
tests: passed=8 failed=0 skipped=0 elapsedMs=6.0207