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/Scripts/Runtime/UnityAudioEngineAmbisonicSource.cs

@@ -0,0 +1,40 @@
+//
+// 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.
+//
+
+using System;
+using UnityEngine;
+
+namespace SteamAudio
+{
+    public sealed class UnityAudioEngineAmbisonicSource : AudioEngineAmbisonicSource
+    {
+        AudioSource mAudioSource = null;
+
+        public override void Initialize(GameObject gameObject)
+        {
+            mAudioSource = gameObject.GetComponent<AudioSource>();
+        }
+
+        public override void UpdateParameters(SteamAudioAmbisonicSource ambisonicSource)
+        {
+            if (!mAudioSource)
+                return;
+
+            var index = 0;
+            mAudioSource.SetAmbisonicDecoderFloat(index++, (ambisonicSource.applyHRTF) ? 1.0f : 0.0f);
+        }
+    }
+}