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/Editor/SteamAudioUnityEditor.asmdef

@@ -0,0 +1,17 @@
+{
+    "name": "SteamAudioUnityEditor",
+    "references": [
+        "SteamAudioUnity"
+    ],
+    "includePlatforms": [
+        "Editor"
+    ],
+    "excludePlatforms": [],
+    "allowUnsafeCode": false,
+    "overrideReferences": false,
+    "precompiledReferences": [],
+    "autoReferenced": true,
+    "defineConstraints": [],
+    "versionDefines": [],
+    "noEngineReferences": false
+}