Migrate shaders to URP and add loading screen

Reworks multiple shaders and materials for URP compatibility and adds a Loading screen/scene and a few model imports. Key changes:
- Converted ProBuilder standard vertex-color shader and several particle/surface shaders to URP HLSL passes (UniversalForward, ShadowCaster, Depth, DepthNormals) preserving base texture * tint * vertex color, normal map and PBR parameters.
- Updated particle SurfaceShader_VC to a URP forward pass and simplified lighting to use URP shader library helpers.
- Updated materials (landMark, tile) to point to project textures, adjust keywords/flags (e.g. XRMotionVectorsPass, disable ShadowCaster for one), tweak tiling and base color values.
- Added a Loading screen UI (UXML, USS) and LoadingScreenController.cs plus a new Loading scene and scene metadata.
- Imported new FBX assets (T-Pose, Untitled) and updated Editor build settings / project settings to include the new Loading scene.
These changes migrate rendering code to the Universal Render Pipeline and add a basic loading UI/scene, while updating materials and project settings accordingly.

Assets/Game/Menus/MainMenu/MainMenuController.cs

@@ -47,8 +47,8 @@ public class MainMenuController : MonoBehaviour
     private void OnPlayClicked()
     {
         PlayClickSound();
-        Debug.Log("¡BWAAAH! Iniciando juego...");
-        // SceneManager.LoadScene("GameScene");
+        Debug.Log("Cargando SampleScene...");
+        SceneManager.LoadScene("Loading");
     }
 
     private void OnSettingsClicked()