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>

ProjectSettings/ProjectSettings.asset

@@ -833,7 +833,11 @@ PlayerSettings:
   webGLCloseOnQuit: 0
   webWasm2023: 0
   webEnableSubmoduleStrippingCompatibility: 0
-  scriptingDefineSymbols: {}
+  scriptingDefineSymbols:
+    Android: STEAMAUDIO_ENABLED
+    Standalone: STEAMAUDIO_ENABLED
+    WebGL: STEAMAUDIO_ENABLED
+    iPhone: STEAMAUDIO_ENABLED
   additionalCompilerArguments: {}
   platformArchitecture: {}
   scriptingBackend: