Add Pause Menu UI, terrains, and X Bot model

Add a new Pause Menu UI (UXML, USS, TSS, asset/meta) and related UI folder metadata. Add new terrain assets and TerrainData files, plus lighting settings for the 2.5d demo scene. Import a new character model (X Bot.fbx) with its importer meta and new characters folder. Update SampleScene and LFS pointers, and refresh render pipeline settings (Mobile RP & URP global settings) and project/package settings. Tweak multiple material files: align _Color with BaseColor, add SHADOWCASTER to disabled shader passes for Ellen materials, and assign a missing main texture for Vincent_Hero_High_diffuse.

Assets/UI/PauseMenu.uss.meta

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