fix: make TestLevel load on 4.6.3; build geometry from GDScript

- test_level.tscn: minimal Node3D root (zero SubResource references)
- debug/test_level_builder.gd: runtime floor/walls/lighting/camera/UI
  shapes/meshes created from BoxShape3D/BoxMesh — no .tscn subresources
- project.godot: revert features to 4.6 (installed build is 4.6.3)

project.godot

@@ -11,7 +11,7 @@ config_version=5
 [application]
 
 config/name="Papaya-Shooter"
-config/features=PackedStringArray("4.2", "Forward Plus")
+config/features=PackedStringArray("4.6", "Forward Plus")
 config/icon="res://icon.svg"
 
 [physics]