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>

.gitignore

@@ -103,3 +103,6 @@ crashlytics-build.properties
 
 # DeepBump CLI (local ML tool: cloned repo + machine-local venv, see TextureNormalMapGenerator.cs)
 /Tools/DeepBump/
+
+# Steam Audio download/extract staging (the imported plugin lives in Assets/Plugins/SteamAudio)
+/Tools/SteamAudio/