Content/authoring: concealment colliders, scene + material tweaks, ignore third-party asset packs

- Generated concealment colliders for The Room (Fortuny ON base/top)
- Main + The Room scene edits, Wall_White material variants
- ProjectSettings churn (PackageManager, ProBuilder settings)
- .gitignore: exclude imported Allsky / ArchVizPRO Vol.6 / Isle of Assets packs

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Assets/Scenes/Main.unity

@@ -10732,13 +10732,13 @@ MonoBehaviour:
     DebugModeWS: 0
     HBuffer: 0
     VolumetricDebug: 1
-    _intensity: 1.5
+    _intensity: 2
     AmbientOverride: 1
     MetallicIndirectFallback: 0
     _renderScale: 1
     TracingMode: 0
     _rayCountMode: 0
-    _rayLength: 50
+    _rayLength: 100
     Multibounce: 1
     ExcludeCastingMask:
       serializedVersion: 0