Audio: per-SoundEffect occlusion via Steam Audio + 3D flag refactor

- New occlusion opt-in on SoundEffect: spatialized instances get a
  SteamAudioSource with raycast occlusion + frequency-dependent
  transmission (walls muffle instead of hard-mute)
- Replace SoundEffectDistanceRange asset indirection with an explicit
  _3d bool (user refactor); rolloff/distance now left to defaults
- Hey.asset: occlusion enabled

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Assets/Scripts/Audio/SoundEffectInstance.cs

@@ -23,16 +23,19 @@ namespace Prototype
                 audioSource.volume = options.GetVolume();
                 audioSource.pitch = options.GetPitch();
 
-                var range = options.GetDistanceRange();
-
-                audioSource.spatialBlend = range.x == 0 && range.y == 0 ? 0 : 1;
+                audioSource.spatialBlend = options.Get3D() ? 1 : 0;
                 audioSource.spatialize = options.GetSpatialize() && audioSource.spatialBlend > 0;
-                audioSource.rolloffMode = AudioRolloffMode.Linear;
-                audioSource.minDistance = range.x;
-                audioSource.maxDistance = range.y;
                 audioSource.loop = options.GetLoop();
             }
 
+            if (audioSource.spatialize && options.GetOcclusion())
+            {
+                var steamAudioSource = gameObject.AddComponent<SteamAudio.SteamAudioSource>();
+                steamAudioSource.occlusion = true;
+                steamAudioSource.transmission = true;
+                steamAudioSource.transmissionType = SteamAudio.TransmissionType.FrequencyDependent;
+            }
+
             if (!options.ShouldDestroyOnLoad())
             {
                 DontDestroyOnLoad(this);