feat(editor): add UI renderer pipeline and shader

Add UiRenderer with wgpu render pipeline for 2D UI overlay rendering.
Includes WGSL shader with orthographic projection, alpha blending,
and R8Unorm font atlas sampling. Font pixel (0,0) set to white for
solid-color rect rendering via UV (0,0).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/voltex_editor/Cargo.toml

@@ -5,3 +5,4 @@ edition = "2021"
 
 [dependencies]
 bytemuck = { workspace = true }
+wgpu = { workspace = true }