refactor: do not nest asset namespaces in game namespaces

* Duplicated namespace names are kind of annoying

test/ObjWritingTests/Game/T6/Material/MaterialJsonDumperT6Test.cpp

@@ -472,7 +472,7 @@ namespace
         AssetPoolDynamic<Material> materialPool(0);
         GivenMaterial("wpc/metal_ac_duct", materialPool, memory);
 
-        material::JsonDumper dumper;
+        material::JsonDumperT6 dumper;
         dumper.DumpPool(context, &materialPool);
 
         const auto* file = mockOutput.GetMockedFile("materials/wpc/metal_ac_duct.json");