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>

Cargo.lock

@@ -492,6 +492,7 @@ dependencies = [
  "log",
  "pollster",
  "voltex_editor",
+ "voltex_math",
  "voltex_platform",
  "voltex_renderer",
  "wgpu",
@@ -2108,6 +2109,7 @@ version = "0.1.0"
 dependencies = [
  "bytemuck",
  "voltex_math",
+ "voltex_renderer",
  "wgpu",
 ]