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/AudioManager.asset

@@ -12,8 +12,12 @@ AudioManager:
   m_DSPBufferSize: 1024
   m_VirtualVoiceCount: 512
   m_RealVoiceCount: 32
-  m_SpatializerPlugin: 
-  m_AmbisonicDecoderPlugin: 
+  m_EnableOutputSuspension: 1
+  m_SpatializerPlugin: Steam Audio Spatializer
+  m_AmbisonicDecoderPlugin: Steam Audio Ambisonic Decoder
   m_DisableAudio: 0
   m_VirtualizeEffects: 1
   m_RequestedDSPBufferSize: 0
+  m_AudioFoundation: 0
+  m_OutputChannelLayout: 2
+  m_OutputSamplingRate: 48000