feat: rebuild Tutorial scene avec ProBuilder, matériaux gel/terrain

- Scène Tutorial reconstruite de zéro : sol ProBuilder 120x120, murs, rampes
- Zones race : 5 checkpoints (CP0→Finish) avec BoxCollider + CheckpointTrigger
- Gel pads : GelOrange (boost), GelBleu (bounce), GelViolet (sticky)
- Plateforme centrale pour mode teams (CaptureZone)
- Nouveaux matériaux URP : FloorMat, WallMat, CheckpointMat, GelOrangeMat, GelBleuMat, GelVioletMat, PlatformMat

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

game/Assets/Material/FloorMat.mat

@@ -0,0 +1,84 @@
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