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/LoadingScreen/LoadingScreen.uss

@@ -0,0 +1,47 @@
+/* Contenedor Principal responsivo */
+.loading-container {
+    width: 100%;
+    height: 100%;
+    flex-grow: 1;
+    background-color: #000000;
+    align-items: flex-end;
+    justify-content: flex-end;
+    position: relative;
+}
+
+/* Texto de Carga */
+.loading-text {
+    font-size: 50px;
+    color: #ffffff;
+    -unity-font-style: bold;
+    margin-right: 40px;
+    margin-bottom: 40px;
+    
+    /* 1. Hacemos la caja más ancha para que quepa "CARGANDO..." con la fuente Komika */
+    width: 350px; 
+    
+    /* 2. Anclamos el texto a la izquierda de la caja */
+    -unity-text-align: middle-left; 
+    
+    /* 3. Evitamos estrictamente que el texto salte a la siguiente línea */
+    white-space: nowrap; 
+}
+
+/* Contenedor de la Nave */
+.spaceship-icon {
+    /* 1. Quitamos position: absolute para que respete el flujo y baje a la esquina */
+    position: relative;
+    
+    /* 2. Tamaño ajustado */
+    width: 250px;
+    height: 250px;
+    
+    /* 3. Márgenes para alinearla con el texto (mismo margin-left que el texto) */
+    margin-left: 40px;
+    
+    /* 4. Un pequeño margen abajo para que no pegue con las letras */
+    margin-bottom: -20px; 
+    
+    /* Ajuste de imagen */
+    -unity-background-scale-mode: scale-to-fit;
+}