Import Poly Haven materials and scene/authoring updates

Three imported PBR texture sets with generated URP/Lit materials
(Brick 4, Brick Floor 002, Brick Wall 003), scene churn in Main and
The Room, ProBuilder palette/settings updates.

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

Assets/Scenes/Main.unity

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