Add Steam Audio 4.8.1 spatializer plugin

Official Unity package under Assets/Plugins/SteamAudio; enabled as
Spatializer + Ambisonic Decoder in the audio settings (HRTF: sounds
behind the listener are filtered/muffled directionally). Download
staging Tools/SteamAudio/ gitignored.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Assets/Plugins/SteamAudio/Binaries/iOS/SteamAudioUnityAppController.h

@@ -0,0 +1,23 @@
+//
+// Copyright 2017-2023 Valve Corporation.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+
+#import "UnityAppController.h"
+
+@interface SteamAudioUnityAppController : UnityAppController
+
+- (void) preStartUnity;
+
+@end