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/Scenes/Loading.unity

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:33c4ed4fd5722602be7c25ca29d21674d138239cd6d340e0e5aa3857a0ce88ce
+size 12115

Assets/Scenes/Loading.unity.meta

@@ -0,0 +1,7 @@
+fileFormatVersion: 2
+guid: 878972854fc47d147981f67915c6b8e5
+DefaultImporter:
+  externalObjects: {}
+  userData: 
+  assetBundleName: 
+  assetBundleVariant: 

Assets/Scenes/SampleScene.unity

@@ -1,3 +1,3 @@
 version https://git-lfs.github.com/spec/v1
-oid sha256:045e5a15d9d0971c25d24fec4a150dfe821b29c20c6d2c0ea30df737cb2c3050
-size 115044360
+oid sha256:73e133f452c7d794ad49f4faee987c1509bdc80063f9e98df734dde24ac05e3a
+size 86888977