feat(editor): integrate 3D viewport into editor_demo

Render actual 3D geometry (cubes + ground plane) in the Viewport panel
using the existing mesh_shader.wgsl Blinn-Phong pipeline with orbit
camera controls (left-drag to orbit, middle-drag to pan, scroll to zoom).

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

crates/voltex_editor/Cargo.toml

@@ -6,4 +6,5 @@ edition = "2021"
 [dependencies]
 bytemuck = { workspace = true }
 voltex_math = { workspace = true }
+voltex_renderer = { workspace = true }
 wgpu = { workspace = true }