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>

.gitignore

@@ -106,3 +106,12 @@ crashlytics-build.properties
 
 # Steam Audio download/extract staging (the imported plugin lives in Assets/Plugins/SteamAudio)
 /Tools/SteamAudio/
+
+/Assets/Allsky.meta
+/Assets/Allsky/
+
+/Assets/ArchVizPRO Interior Vol.6.meta
+/Assets/ArchVizPRO Interior Vol.6/
+
+/Assets/Isle of Assets.meta
+/Assets/Isle of Assets/