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 Kit Controller/Shaders/runShader.shader

@@ -1,34 +1,66 @@
-Shader "Custom/prueba" {
-Properties {
-    _Color ("Main Color", Color) = (1,1,1,1)
-    _SpecColor ("Spec Color", Color) = (1,1,1,0)
-    _Emission ("Emissive Color", Color) = (0,0,0,0)
-    _Shininess ("Shininess", Range (0.1, 1)) = 0.7
-    _MainTex ("Base (RGB) Trans (A)", 2D) = "white" {}
-}
- 
-SubShader {
-    Tags {"RenderType"="Transparent" "Queue"="Transparent"}
-    // Render into depth buffer only
+Shader "Custom/URP_prueba"
+{
+    Properties {
+        _Color ("Main Color", Color) = (1,1,1,1)
+        _SpecColor ("Spec Color", Color) = (1,1,1,0)
+        _Emission ("Emissive Color", Color) = (0,0,0,0)
+        _Shininess ("Shininess", Range (0.1, 1)) = 0.7
+        _MainTex ("Base (RGB) Trans (A)", 2D) = "white" {}
+    }
+    SubShader {
+        Tags {"RenderType"="Transparent" "Queue"="Transparent"}
+        LOD 200
         Pass {
-        ColorMask 0
-    }
-    // Render normally
-    Pass {
-        ZWrite Off
-        Blend SrcAlpha OneMinusSrcAlpha
-        ColorMask RGB
-        Material {
-            Diffuse [_Color]
-            Ambient [_Color]
-            Shininess [_Shininess]
-            Specular [_SpecColor]
-            Emission [_Emission]
-        }
-        Lighting On
-        SetTexture [_MainTex] {
-            Combine texture * primary DOUBLE, texture * primary
+            Name "FORWARD"
+            ZWrite Off
+            Blend SrcAlpha OneMinusSrcAlpha
+            ColorMask RGB
+            HLSLPROGRAM
+            #pragma vertex vert
+            #pragma fragment frag
+            #pragma multi_compile_fog
+            #include "Packages/com.unity.render-pipelines.universal/ShaderLibrary/Core.hlsl"
+
+            struct Attributes {
+                float4 positionOS : POSITION;
+                float2 uv         : TEXCOORD0;
+            };
+
+               struct Varyings {
+                   float4 positionHCS : SV_POSITION;
+                   float2 uv          : TEXCOORD0;
+                   float fogFactor    : TEXCOORD1;
+               };
+
+            CBUFFER_START(UnityPerMaterial)
+                float4 _Color;
+                float4 _SpecColor;
+                float4 _Emission;
+                float _Shininess;
+            CBUFFER_END
+
+            TEXTURE2D(_MainTex); SAMPLER(sampler_MainTex);
+
+            Varyings vert(Attributes IN)
+            {
+                Varyings OUT;
+                OUT.positionHCS = TransformObjectToHClip(IN.positionOS.xyz);
+                OUT.uv = IN.uv;
+                OUT.fogFactor = ComputeFogFactor(OUT.positionHCS.z / OUT.positionHCS.w);
+                return OUT;
+            }
+
+            float4 frag(Varyings IN) : SV_Target
+            {
+                float4 albedo = SAMPLE_TEXTURE2D(_MainTex, sampler_MainTex, IN.uv) * _Color;
+                float3 emission = _Emission.rgb;
+                float3 specular = _SpecColor.rgb * _Shininess;
+                float3 color = albedo.rgb + emission + specular;
+                float alpha = albedo.a;
+                color = MixFog(color, IN.fogFactor);
+                return float4(color, alpha);
+            }
+            ENDHLSL
         }
     }
-}
 }