Expose read-only SceneId and callback-free SyncVar setter for state serialization

[desuqcafe]

External systems that serialize/restore SyncType state (save/load, host migration, rollback) currently require reflection to access two internal members.

SyncVar<T> — setting value without triggering OnChange

When restoring serialized state, OnChange callbacks shouldn't fire, subscribers would receive stale previous values and potentially corrupt their own state.

Current workaround (reflection):

FieldInfo field = syncVarType.GetField("_value", BindingFlags.Instance | BindingFlags.NonPublic);
field.SetValue(syncVarInstance, restoredValue);

SetValue(value, calledByUser: false) doesn't work here because it still invokes InvokeOnChange, and the IsServerStarted path skips UpdateValues entirely on clientHost.

I saw there is ResetState does _value = _initialValue directly. Something like:

public void SetValueDirect(T value)
{
    _value = value;
}

Or using UpdateValues(value) to also update the interpolator and prevent visual snapping.

NetworkObject.SceneId (read-only access)

IsSceneObject is public, but correlating scene objects across sessions requires the actual value. Currently accessed via reflection:

FieldInfo field = typeof(NetworkObject).GetField("SceneId", BindingFlags.Instance | BindingFlags.NonPublic);
ulong sceneId = (ulong)field.GetValue(nob);

A public getter would be sufficient:

public ulong GetSceneId() => SceneId;