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>

examples/editor_demo/Cargo.toml

@@ -7,6 +7,7 @@ edition = "2021"
 voltex_platform.workspace = true
 voltex_renderer.workspace = true
 voltex_editor.workspace = true
+voltex_math.workspace = true
 wgpu.workspace = true
 winit.workspace = true
 bytemuck.workspace = true