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>

ProjectSettings/Packages/com.unity.probuilder/Settings.json

@@ -179,7 +179,7 @@
             {
                 "type": "UnityEngine.ProBuilder.SelectMode, Unity.ProBuilder, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null",
                 "key": "s_SelectMode",
-                "value": "{\"m_Value\":1}"
+                "value": "{\"m_Value\":4}"
             },
             {
                 "type": "UnityEngine.ProBuilder.ExtrudeMethod, Unity.ProBuilder, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null",